AU2002346577A1 - Sulfur containing lubricating oil additive system particularly useful for natural gas fueled engines - Google Patents

Description

1 SULFUR CONTAINING LUBRICATING OIL 2 ADDITIVE SYSTEM PARTICULARLY USEFUL FOR 3 NATURAL GAS FUELED ENGINES 4 5 BACKGROUND 6 This invention relates to lubricating oil comprising a combination of a 7 hindered phenol and Group II, Ill and IV base oil. The lubricating oil of this 8 invention may be used in any manner, however its enhanced properties make 9 it particularly applicable for use in engines fueled by natural gas. 10 Natural gas has a higher specific heat content than liquid hydrocarbon 11 fuels and therefore it burns hotter than liquid hydrocarbon fuels under typical 12 conditions. In addition, since it is already a gas, natural gas does not cool 13 intake air by evaporation as liquid hydrocarbon fuel droplets do. Furthermore, 14 many natural gas fueled engines are run either at or near stoichiometric 15 conditions, at which less excess air is available to dilute and cool combustion 16 gases. As a result, natural gas fueled engines generate higher combustion 17 gas temperatures than engines burning liquid hydrocarbon fuels. Since the 18 rate of formation of NOx increases exponentially with temperature, natural gas 19 fueled engines may generate NOx concentrations high enough to cause 20 severe nitration of lubricating oil. 21 In most cases, natural gas fueled engines are used continuously at 22 70 to 100% load, whereas an engine operating in vehicular service may only 23 spend 50% of its time at full load. Lubricating oil drain intervals may vary in 24 vehicular service, but are typically shorter than those for natural gas fueled 25 engines. 26 It is important to ensure the reliability of natural gas fueled engines 27 because natural gas fueled engines may be located in remote areas where 28 service is not readily available. Lubricating oil used in natural gas engines 29 therefore requires high resistance to oxidation and nitration. 30 Good valve wear control is important for keeping engine operating 31 costs down and may be achieved by providing the proper amount and 32 composition of ash. Minimizing combustion chamber deposits and spark plug 33 fouling are also considerations in setting the ash content and composition in 34 these oils. Lubricating oil ash levels are limited, so detergents must be -1- 1 carefully selected to minimize piston deposits and ring sticking. Good wear 2 protection is required to prevent scuffing and corrosion. 3 If lubricating oils for natural gas fueled engines are not formulated to 4 handle typical environments for those engines, the lubricating oil will 5 deteriorate rapidly during use. This deterioration will typically cause the 6 lubricating oil to thicken, which results in engine sludge, piston deposits, oil 7 filter plugging, and in severe cases, accelerated ring and liner wear. 8 The general industry approach to reduce deterioration of lubricating oil 9 and the resultant engine sludge, piston deposits, oil filter plugging and o10 accelerated ring and liner wear is to add antioxidants such as hindered phenols 11 as well as diphenyl amines and sulfurized compounds. Increasing the amount 12 of these antioxidants in lubricating oil is increasingly effective to avoid 13 lubricating oil deterioration. But at some point the solubility limit of the additional 14 antioxidant reaches maximum effectiveness and at times further addition of 15 antioxidant may even detrimentally affect piston deposit control. 16 While it is no surprise that increasing the amount of antioxidant is 17 effective in increasing the life of lubricating oil, this invention provides a s18 method to increase the life of lubricating oil with out necessarily increasing the 19 amount of antioxidant. 20 21 SUMMARY 22 The lubricating oil of this invention may comprise a minor amount of one or 23 more hindered phenols of the general formula: 24 O HO CH 2 - S - CH 2 - C - O - R 25-(1) 26 27 wherein R is a C 7

-C

9 alkyl group 28 and a major amount of at least one of Group II, Ill and IV base oils. More 29 specifically, the lubricating oil of this invention may comprise about -2- 1 0.20 wt. % to about 3 wt. % of one or more hindered phenols having this 2 general formula. Liquid hindered phenols are preferred. Unless otherwise 3 specified the term "wt. %" as used herein means wt. % of lubricating oil. One 4 embodiment of this invention comprises a lubricating oil of Claim 1 having a 5 total base number of about 2.15 milligrams Potassium Hydroxide per gram of 6 sample (mg KOH/gr) to about 8.88 mg KOH/gr as determined by 7 ASTM D 2896. One embodiment of this invention comprises a lubricating oil 8 having a total ash content of about 0.10 wt. % to about 1.50 wt. % as 9 determined by ASTM D874. Lubricating oil of this invention may have less 10 than 4000 ppm sulfur. One embodiment of this invention comprises combining 11 the hindered phenol of the invention with the base oil in any order and mixing. 12 Another embodiment of this invention comprises a method of lubricating 13 engines comprising contacting one or more engines with the lubricating oil of 14 this invention. Lubricating oil of this invention may comprise a major amount 15 of at least one of Group II, III and IV base oil and a minor amount of 16 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate of formula (1), above, also 17 known as acetic acid, [[[3,5-bis(1,1-dimethyl ethyl)-4 18 hydroxyphenyl]methyl]thio]-, C 7

-C

9 alkyl ester. Lubricating oil of this invention 19 may comprise about 0.20 wt. % to about 3 wt. 2-(4-hydroxy-3, 5-di-t-butyl 20 benzyl thiol) acetate, preferably about 0.6 wt % to about 2.5 wt. %. 2-(4 21 hydroxy-3, 5-di-t-butyl benzyl thiol) acetate may be liquid. One embodiment of 22 this invention may comprise an additive formulation comprising 2-(4-hydroxy 23 3, 5-di-t-butyl benzyl thiol) acetate; one or more dispersants; one or more 24 wear inhibitors; and one or more detergents. Lubricating oil of this invention 25 may comprise about 1 wt. % to about 8 wt. % of one or more dispersants, 26 about 1 wt. % to about 8.5 wt. % of one or more detergents, about 0.2 wt. % 27 to about 1.5 wt. % of one or more wear inhibitors, about 0.5 wt. % to about 3 28 wt. % 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate, and about 40 wt. % to 29 about 97 wt. % of at least one of Group II, Ill and IV base oil or preferably 30 about 80 wt. % to about 97 wt. % of at least one of Group II, Ill and IV base oil 31 or more preferably about 60 wt. % to about 97 wt. % of at least one of 32 Group II, III and IV base oil. Lubricating oil of this invention may comprise 33 about 1.25 wt. % to about 6 wt. % of one or more dispersants; about 2 wt. % to -3- 1 about 6 wt. % of one or more detergents; about 0.3 wt. % to about 0.8 wt. % of 2 one or more wear inhibitors, about 0.6 to about 2.5 wt. % 2-(4-hydroxy-3, 5-di-t 3 butyl benzyl thiol) acetate and about 40 wt. % to about 97 wt. % of at least 4 one of Group II, Ill and IV base oil or preferably about 80 wt. % to about 97 wt. 5 % of at least one of Group II, Ill and IV base oil or more preferably about 60 6 wt. % to about 97 wt. % of at least one of Group II, Ill and IV base oil. 7 8 DETAILED DESCRIPTION OF THE INVENTION 9 This invention provides lubricating oil that may be used in any engine, 10 but that has exhibited a surprisingly long life when tested in a natural gas fueled 11 engine. 12 The lubricating oil of this invention may comprise one or more of the 13 hindered phenols described herein and Group II, Ill and IV base oils. A 14 preferred lubricating oil of this invention comprises a major amount of one or 15 more base oils from Groups II through IV and a minor amount of the hindered 16 phenols described herein. The term "major amount" when used herein means 17 more than 40 wt. %. The term "minor amount" when used herein means less 18 than 20 wt. %. 19 One embodiment of this invention comprises an additive formulation 20 comprising one or more of the hindered phenols described herein, one or more 21 dispersants, one or more detergents and one or more wear inhibitors. 22 A preferred lubricating oil of this invention may comprise a major amount 23 of base oils from Group II through Group IV, a minor amount of one or more of 24 the additive formulations comprising the hindered phenols described herein, 25 one or more detergents, one or more dispersants and one or more wear 26 inhibitors. 27 Another embodiment of this invention comprises lubricating oils 28 comprising additive formulations comprising the hindered phenols described 29 herein. 30 Preferred lubricating oil of this invention may comprise the hindered 31 phenols described herein and Group II through IV base oils in a formulation 32 that has about 0.10 wt. % to about 1.50 wt. % ash in the finished lubricating oil 33 and more preferably about 0.3 wt. % ash to about 0.95 wt. % ash in the finished -4- 1 lubricating oil as determined by ASTM D874. When ash contents are discussed 2 herein, the ash contents were determined by ASTM D874. 3 One embodiment of the lubricating oil of this invention may have a 4 Total Base Number (TBN) of about 2.15 milligrams Potassium Hydroxide 5 per gram of sample (mg KOH/gr) to about 8.88 mg KOH/gr. A more preferable 6 embodiment would have a TBN from about 3 mg KOH/gr to about 7 8 mg KOH/gr. Unless otherwise specified, TBNs, as used herein, are s determined by using the method ASTM D2896. 9 The lubricating oil of this invention may have a sulfur content of less than 10 4000 ppm or 0.4 wt. %. 11 Another embodiment of this invention comprises methods of making the 12 lubricating oils of this invention or the additive formulations of this invention by 13 combining the components with agitation until all components are mixed. The 14 ingredients may be combined in any order and at a temperature sufficient to 15 blend the components but not high enough to degrade the components. A 16 temperature of about 120 degrees F. to about 160 degrees F. may be used. 17 Heating may occur at any time during the process. is Another embodiment of this invention comprises the method of 19 lubricating one or more engines by contacting one or more of the lubricating oils 20 of this invention with one or more engines. 21 Another embodiment of this invention comprises the method of 22 lubricating one or more natural gas engines by contacting one or more of the 23 lubricating oils of this invention with one or more natural gas engines. 24 Another embodiment of this invention comprises the method of 25 lubricating one or more engines by lubricating one or more of the lubricating oils 26 of this invention with one or more engines. 27 Another embodiment of this invention comprises the method of 28 lubricating one or more natural gas engines by lubricating one or more of the 29 lubricating oils of this invention with one or more natural gas engines. 30 The lubricating oil of this invention has shown a surprisingly long life in a 31 natural gas fueled engine when the hindered phenols described herein are 32 used in an additive formulation with Group II, Ill and IV base oils over that seen 33 when the hindered phenols described herein were used in a similar additive -5- 1 formulation with Group I base oils or when other types of hindered phenols 2 were used in a similar additive formulation with Group I or Group 11 base oils. 3 The surprising long life exhibited by the lubricating oil of this invention may be 4 the result of a synergistic effect of the Group II, Ill and IV base oils with the 5 hindered phenols described herein and/or a synergistic effect of the Group II, Ill 6 and IV base oils and the additive formulation comprising the hindered phenols 7 described herein. 8 9 I. BASE OIL 10 Base Oil as used herein is defined as a base stock or blend of base 11 stocks. Base Stock as used herein is defined as a lubricant component that is 12 produced by a single manufacturer to the same specifications (independent of 13 feed source or manufacturers location): that meets the same manufacturer's 14 specification; and that is identified by a unique formula, product identification 15 number, or both. Base stocks may be manufactured using a variety of 16 different processes including but not limited to distillation, solvent refining, 17 hydrogen processing, oligomerization, esterification, and rerefining. Rerefined 18 stock shall be substantially free from materials introduced through 19 manufacturing, contamination, or previous use. The base oil of this invention 20 may be any natural or synthetic lubricating base oil fraction particularly those 21 having a kinematic viscosity at 100 degrees Centigrade (C) and about 22 5 centistokes (cSt) to about 20 cSt, preferably about 7 cSt to about 23 16 cSt, more preferably about 9 cSt to about 15 cSt. Hydrocarbon synthetic 24 oils may include, for example, oils prepared from the polymerization of 25 ethylene, i.e., polyalphaolefin or PAO, or from hydrocarbon synthesis 26 procedures using carbon monoxide and hydrogen gases such as in a 27 Fisher-Tropsch process. A preferred base oil is one that comprises little, if 28 any, heavy fraction; e.g., little, if any, lube oil fraction of viscosity 20 cSt or 29 higher at 100 degrees C. 30 The base oil may be derived from natural lubricating oils, synthetic 31 lubricating oils or mixtures thereof. Suitable base oil includes base stocks 32 obtained by isomerization of synthetic wax and slack wax, as well as 33 hydrocrackate base stocks produced by hydrocracking (rather than solvent -6- 1 extracting) the aromatic and polar components of the crude. Suitable base oils 2 include those in API categories II, Ill, IV and V. Saturates levels and viscosity 3 indices for Group I, II and III base oils are listed in Table 1. Group IV base oils 4 are polyalphaolefins (PAO). Group V base oils include all other base oils not 5 included in Group I, II, Ill, or IV. Suitable base oils for use in this invention 6 include those in API categories II, Ill and IV as defined in 7 API Publication 1509,14th Edition, Addendum I, December 1998. A summary 8 of the characteristics of Group 1, II, and III base oil is presented in Table I. 9 Though Group II, Ill and IV base oils are preferred for use in this invention, o10 these preferred Group II, Ill and IV base oils may be prepared by combining 11 one or more of Group I, II, Ill, IV and V base stocks or base oils. 12 13 TABLE 1 14 Saturates, Sulfur and 15 Viscosity Index of Group I, II and Ill Base Stocks Group Saturates Viscosity Index (As determined by ASTM D 2007) (As determined by Sulfur ASTM D 4294, ASTM D (As determined by ASTM D 2270) 4297 or ASTM D 3120) I Less than 90 % saturates and/or Greater than Greater than or equal to to 0.03 % sulfur 80 and less than 120 II Greater than or equal to 90 % saturates and Greater than or equal to less than or equal to 0.03 % sulfur 80 and less than 120 III Greater than or equal to 90 % saturates and Greater than or equal to 120 less than or equal to 0.03% sulfur 16 17 Natural lubricating oils may include animal oils, vegetable oils is (e.g., rapeseed oils, castor oils and lard oil), petroleum oils, mineral oils, and 19 oils derived from coal or shale. 20 Synthetic oils may include hydrocarbon oils and halo-substituted 21 hydrocarbon oils such as polymerized and inter-polymerized olefins, 22 alkylbenzenes, polyphenyls, alkylated diphenyl ethers, alkylated diphenyl 23 sulfides, as well as their derivatives, analogues and homologues thereof, and 24 the like. Synthetic lubricating oils also include alkylene oxide polymers, 25 interpolymers, copolymers and derivatives thereof wherein the terminal 26 hydroxyl groups have been modified by esterification, etherification, etc. -7- 1 Another suitable class of synthetic lubricating oils comprises the esters of 2 dicarboxylic acids with a variety of alcohols. Esters useful as synthetic oils 3 also include those made from C5 to C12 monocarboxylic acids and polyols and 4 polyol ethers. Tri-alkyl phosphate ester oils such as those exemplified by 5 tri-n-butyl phosphate and tri-iso-butyl phosphate are also suitable for use as 6 base oils. 7 Silicon-based oils (such as the polyakyl-, polyaryl-, polyalkoxy-, or 8 polyaryloxy-siloxane oils and silicate oils) comprise another useful class of 9 synthetic lubricating oils. Other synthetic lubricating oils include liquid esters 10 of phosphorus-containing acids, polymeric tetrahydrofurans, polyalphaolefins, 11 and the like. 12 The base oil may be derived from unrefined, refined, rerefined oils, or 13 mixtures thereof. Unrefined oils are obtained directly from a natural source or 14 synthetic source (e.g., coal, shale, or tar sand bitumen) without further 15 purification or treatment. Examples of unrefined oils include a shale oil 16 obtained directly from a retorting operation, a petroleum oil obtained directly 17 from distillation, or an ester oil obtained directly from an esterification process, 18 each of which may then be used without further treatment. Refined oils are 19 similar to the unrefined oils except that refined oils have been treated in one 20 or more purification steps to improve one or more properties. Suitable 21 purification techniques include distillation, hydrocracking, hydrotreating, 22 dewaxing, solvent extraction, acid or base extraction, filtration, and 23 percolation, all of which are known to those skilled in the art. Rerefined oils 24 are obtained by treating used oils in processes similar to those used to obtain 25 the refined oils. These rerefined oils are also known as reclaimed or 26 reprocessed oils and often are additionally processed by techniques for 27 removal of spent additives and oil breakdown products. 28 Base oil derived from the hydroisomerization of wax may also be used, 29 either alone or in combination with the aforesaid natural and/or synthetic base 30 oil. Such wax isomerate oil is produced by the hydroisomerization of natural or 31 synthetic waxes or mixtures thereof over a hydroisomerization catalyst. 32 It is preferred to use a major amount of base oil in the lubricating oil of 33 - this invention. A major amount of base oil as defined herein comprises -8- 1 40 wt. % or more. Preferred amounts of base oil comprise about 40 wt. % to 2 about 97 wt. % of at least one of Group II, Ill and IV base oil or preferably about 3 80 wt. % to about 97 wt. % of at least one of Group II, Ill and IV base oil or 4 more preferably about 60 wt. % to about 97 wt. % of at least one of 5 Group II, Ill and IV base oil. (When wt. % is used herein, it is referring to 6 wt. % of the lubricating oil unless otherwise specified.) A more preferred 7 embodiment of this invention may comprise an amount of base oil that 8 comprises about 85 wt. % to about 95 wt. % of the lubricating oil. 9 A preferred lubricating oil of this invention may comprise Group II base 10 oil. Preferred base oils may comprise base oil that is commercially available 11 from Chevron Corporation in San Ramon, California, 12 Pennzoil Quaker State Company in Houston, Texas, Conoco in Houston, 13 Texas, Motiva Enterprises in Houston, Texas, ExxonMobil in Irving, Texas 14 and Petro Canada Lubricants in Mississauga, Ontario Canada. Other 15 base oils useful in this invention may be commercially available throughout 16 the world from other base oil suppliers. 17 18 II. ADDITIVE FORMULATION 19 When incorporated in lubricating oil, the additive formulation of this 20 invention provides enhanced oxidation inhibition, nitration inhibition, total base 21 retention, reduction in acid formation and reduction percent viscosity increase 22 of lubricating oil. 23 One embodiment of the additive formulation of this invention may 24 comprise of one or more dispersants, one or more detergents, one or more 25 wear inhibitors and one or more hindered phenols described herein. 26 The lubricating oil of this invention may comprise an additive formulation 27 that provides the lubricating oil with about 1 wt. % to about 8 wt. % of one or 28 more dispersants, about 1 wt. % to about 8.5 wt. % of one or more detergents, 29 about 0.2 wt. % to about 1.5 wt. % of one or more wear inhibitors and about 30 0.2 wt. % to about 3 wt. % and one or more hindered phenols described 31 herein. The additive formulation of this invention may also comprise other 32 additives traditionally used in the lubricating oil industry. -9- 1 Another embodiment of a lubricating oil of this invention may comprise 2 an additive formulation that provides the lubricating oil with about 1.25 wt. % to 3 about 6 wt. % of one or more dispersants, about 2 wt. % to about 6 wt. % of 4 one or more detergents, about 0.3 wt. % to about 0.8 wt. % of one or more 5 wear inhibitors and about 0.6 wt. % to about 2.5 wt. % of one or more hindered 6 phenols described herein. These components make up one embodiment of 7 the additive formulation of this invention. The additive formulation of this 8 invention may also comprise other additives traditionally used in the lubricating 9 oil industry. 10 The additive formulation of this invention may comprise diluent oil. It is 11 known in the art to add diluent oil to additive formulations and this is called 12 "trimming" the additive formulation. A preferred embodiment may be trimmed 13 with any diluent oil typically used in the industry. This diluent oil may be a 14 Group I or above oil. A preferred amount of diluent oil may comprise about 15 4.00 wt.%. 16 17 A. Hindered Phenol Antioxidant s18 One embodiment of this invention comprises one or more hindered 19 phenols having the general formula: 20 O HO -- K CH 2 - S - CH 2 - - O - R 21 22 (1) 23 wherein R is a C 7 -Cg alkyl group. 24 The lubricating oil of this invention may comprise the additive formulation 25 of this invention that provides the lubricating oil with about 0.2 wt. % to about 26 3 wt. % of one or more hindered phenols having the general structural formula 27 (1). Preferred lubricating oils of this invention may comprise an additive 28 formulation that provides the lubricating oil with about 0.6 wt. % to about 29 2.5 wt. % of one or more hindered phenols having the general structural 30 formula (1). -10- 1 Another embodiment of the lubricating oil of this invention may 2 comprise an additive formulation that provides the lubricating oil containing 3 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate having the formula (1) above. 4 The hindered phenol, 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate, also 5 known as acetic acid, [[[3,5-bis(1,1-dimethylethyl)-4 6 hydroxyphenyl]methyl]thio]-, C 7

-C

9 alkyl esters may be available commercially 7 from Ciba Specialty Chemicals at 540 White Plains Road, Tarrytown, NY 8 10591 as IRGANOX L1180. IRGANOX L 118® is a liquid high molecular 9 weight phenolic antioxidant of formula (1) for use in lubricating oils. 10 Liquid hindered phenol is preferred. 11 Lubricating oil of this invention may comprise greater than about 12 0.2 wt. % to more than about 3 wt. % 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) 13 acetate. Preferred lubricating oils of this invention comprise about 0.6 wt. % to 14 about 2.5 wt. % 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate. 15 16 B. Detergent 17 Any detergents commonly used in lubricating oils may be used in this 18 invention. These detergents may or may not be overbased detergents or they 19 may be low, neutral, medium, or high overbased detergents. For example, 20 detergents of this invention may comprise sulfonates, salicylates and 21 phenates. Metal sulfonates, salicylates and phenates are preferred. When the 22 term metal is used with respect to sulfonates, salicylates and phenates herein, 23 it refers to calcium, magnesium, lithium, magnesium, potassium and barium. 24 The detergent may be incorporated into the lubricating oil of this 25 invention in an amount of about 1.0 wt. % to about 8.5 wt. %, preferably from 26 about 2 wt. % to about 6 wt. %. 27 28 C. Dispersant 29 A preferred embodiment of the lubricating oil of this invention may 30 comprise one or more nitrogen containing ashless dispersants of the type 31 generally represented by succinimides (e.g., polyisobutylene succinic 32 acid/anhydride (PIBSA)-polyamine having a PIBSA molecular weight of about 33 700 to 2500). The dispersants may or may not be borated or non-borated. -11 - 1 The dispersant may be incorporated into the lubricating oil of this invention in 2 an amount of about 1 wt. % to about 8 wt. %, more preferably in the amount 3 of about 1.5 wt. % to about 6 wt. %. 4 Preferred dispersants for this invention comprise one or more ashless 5 dispersants having an average molecular weight (mw) of about 1000 to about 6 5000. Dispersants prepared from polyisobutylene (PIB) having a mw of about 7 1000 to about 5000 are such preferred dispersants. 8 A preferred dispersant of this invention may be one or more 9 succinimides. The term "succinimide" is understood in the art to include many o10 of the amide, imide, etc. species that are also formed by the reaction of a 11 succinic anhydride with an amine and is so used herein. The predominant 12 product, however, is succinimide and this term has been generally accepted 13 as meaning the product of a reaction of an alkenyl- or alkyl-substituted 14 succinic acid or anhydride with a polyamine. Alkenyl or alkyl succinimides are 15 disclosed in numerous references and are well known in the art. Certain 16 fundamental types of succinimides and related materials encompassed by the 17 term of art "succinimide" are taught in U.S. Pat. Nos. 2,992,708; 3,018,250; 18 3,018,291; 3,024,237; 3,100,673; 3,172,892; 3,219,666; 3,272,746; 19 3,361,673; 3,381,022; 3,912,764; 4,234,435; 4,612,132; 4,747,965; 20 5,112,507; 5,241,003; 5,266,186; 5,286,799; 5,319,030; 5,334,321; 21 5,356,552; 5,716,912, the disclosures of which are hereby incorporated by 22 reference. 23 This invention may comprise one or more succinimides, which may be 24 either a mono or bis-succinimide. This invention may comprise a lubricating oil 25 involving one or more succinimide dispersants that have or have not been 26 post treated. 27 2s D. Wear inhibitor 29 Wear inhibitors such as metal dithiophosphates (e.g., zinc dialkyl 30 dithiophosphate, ZDDP), metal dithiocarbamates, metal xanthates or 31 tricresylphosphates may be included. Wear inhibitors may be present in the 32 amount of about 0.24 wt. % to 1.5 wt. %, more preferably in the amount of -12- 1 about 0.3 wt. % to about 0.80 wt. %, most preferably in the amount of about 2 0.35 wt. % to about 0.75 wt. % of the lubricating oil. 3 A preferred wear inhibitor is zinc dithiophosphate. Other wear inhibitors 4 that may be included are zinc dialkyldithiophosphate and/or zinc 5 diaryldithiophosphate (ZnDTP). The wear inhibitor may be incorporated into 6 the lubricating oil of this invention in an amount of about 0.2 wt. % to 7 1.5 wt. %, more preferably in the amount of about 0.3 wt. % to about 8 0.8 wt. % of the lubricating oil. These values may include a small amount of 9 hydrocarbon oil that was used in preparing zinc dithiophosphate. Preferred 10 ranges of phosphorus in the finished lubricating oil are about 0.01 wt. % to 11 about 0.11 wt. %, more preferably about 0.02 wt. % to about 0.07 wt. %. 12 The alkyl group in the zinc dialkyldithiophosphate may be, for example, 13 a straight or branched primary, secondary or tertiary alkyl group of about 14 2 to about 18 carbon atoms. Examples of the alkyl groups include ethyl, 15 propyl, iso-propyl, butyl, pentyl, hexyl, heptyl, octyl, decyl, dodecyl, and 16 octadecyl. 17 The alkylaryl group of the zinc dialkylaryldithiophosphate is, for 18 example, a phenyl group having an alkyl group of about 2 to about 19 18 carbon atoms, such as butylphenyl group, nonylphenyl group, and 20 dodecylphenyl group. 21 22 III. OTHER ADDITIVE COMPONENTS 23 The following additive components are examples of some of the 24 components that may be favorably employed in this invention. These 25 examples of additives are provided to illustrate this invention, but they are not 26 intended to limit it: 27 28 A. Antioxidants 29 If desired, additional antioxidants may be used. Other antioxidants may 30 reduce the tendency of mineral oils to deteriorate in service. In addition to the 31 antioxidant systems of this invention, the additive formulation may also 32 include but is not limited to such antioxidants as phenol type (phenolic) 33 oxidation inhibitors, such as 4,4'-methylene-bis(2,6-di-tert-butylphenol), -13- 1 4,4'-bis(2,6-di-tert-butylphenol), 4,4'-bis(2-methyl-6-tert-butylphenol), 2 2,2'-methylene-bis(4-methyl-6-tert-butylphenol), 3 4,4'-butylidene-bis(3-methyl-6-tert-butylphenol), 4 4,4'-isopropylidene-bis(2,6-di-tert-butylphenol), 5 2,2'-methylene-bis(4-methyl-6-nonylphenol), 6 2,2'-isobutylidene-bis(4,6-dimethylphenol), 7 2,2'-methylene-bis(4-methyl-6-cyclohexylphenol), 8 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 9 2,4-dimethyl-6-tert-butyl-phenol, 2,6-di-tert-l-dimethylamino-p-cresol, 10 2,6-di-tert-4-(N,N'-dimethylaminomethylphenol), 11 4,4'-thiobis(2-methyl-6-tert-butylphenol), 12 2,2'-thiobis(4-methyl-6-tert-butylphenol), 13 bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)-sulfide, and 14 bis(3,5-di-tert-butyl-4-hydroxybenzyl). Diphenylamine-type oxidation inhibitors 15 include, but are not limited to, alkylated diphenylamine, 16 phenyl-.alpha.-naphthylamine, and alkylated-.alpha.-naphthylamine. Other 17 types of oxidation inhibitors include metal dithiocarbamate (e.g., zinc 18 dithiocarbamate), and methylenebis (dibutyldithiocarbamate). 19 One embodiment of this invention comprises one or more hindered 20 phenols having the general formula: 21 O HO -Q CH 2 - S - CH 2 - C O - R 22 / 23 (1) 24 wherein R is a C 7

-C

9 alkyl group 25 and no other antioxidant additive. Another embodiment of this invention 26 comprises 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate of formula (1) 27 commercially available from Ciba Specialty Chemicals at 540 White Plains 28 Road, Tarrytown, NY 10591 as IRGANOX LI 18® and no other antioxidant. 29 30 -14- 1 B. Wear Inhibitors 2 In addition to the wear inhibitors mentioned in the additive formulation 3 section, other traditional wear inhibitors may be used. As their name implies, 4 these agents reduce wear of moving metallic parts. Examples of such agents 5 include, but are not limited to, phosphates, phosphites, carbamates, esters, 6 sulfur containing compounds, and molybdenum complexes. 7 8 C. Rust Inhibitors (Anti-Rust Agents) 9 Applicable rust inhibitors include: 10 1. Nonionic polyoxyethylene surface active agents: 11 polyoxyethylene lauryl ether, polyoxyethylene higher alcohol ether, 12 polyoxyethylene nonyl phenyl ether, polyoxyethylene octyl phenyl ether, 13 polyoxyethylene octyl stearyl ether, polyoxyethylene oleyl ether, 14 polyoxyethylene sorbitol monostearate, polyoxyethylene sorbitol mono-oleate, 15 and polyethylene glycol mono-oleate; and 16 2. Other compounds: stearic acid and other fatty acids, 17 dicarboxylic acids, metal soaps, fatty acid amine salts, metal salts of heavy 18 sulfonic acid, partial carboxylic acid ester of polyhydric alcohol, and 19 phosphoric ester. 20 21 D. Demulsifiers 22 Demulsifiers that may be used include addition products of alkylphenol 23 and ethylene oxide, polyoxyethylene alkyl ether, and polyoxyethylene sorbitan 24 ester. 25 26 E. Extreme Pressure Agents (EP Agents) 27 EP Agents that may be used include Zinc dialkyldithiophosphate 28 (primary alkyl, secondary alkyl, and aryl type), sulfurized oils, diphenyl sulfide, 29 methyl trichlorostearate, chlorinated naphthalene, fluoroalkylpolysiloxane, and 30 lead naphthenate. 31 32 F. Friction Modifiers -15- 1 Fatty alcohol, fatty acid, amine, borated ester, and other esters. 2 3 G. Multifunctional Additives 4 Sulfurized oxymolybdenum dithiocarbamate, sulfurized 5 oxymolybdenum organo phosphorodithioate, oxymolybdenum monoglyceride, 6 oxymolybdenum diethylate amide, amine-molybdenum complex compound, 7 and sulfur-containing molybdenum complex compound may be used. 8 9 H. Viscosity Index Improvers 10 Polymethacrylate type polymers, ethylene-propylene copolymers, 11 styrene-isoprene copolymers, hydrated styrene-isoprene copolymers, 12 polyisobutylene, and dispersant type viscosity index improvers may be used. 13 14 I. Pour Point Depressants 15 Polymethyl methacrylate may be used. 16 17 J. Foam Inhibitors is Alkyl methacrylate polymers and dimethyl silicone polymers may be 19 used. 20 21 IV. LUBRICATING OIL FOR NATURAL GAS FUELED ENGINES 22 There is a difference in the lubricating oil requirements for natural gas 23 fueled engines and engines that are fueled by liquid hydrocarbon fuels. The 24 combustion of liquid hydrocarbon fuels such as diesel fuel often results in a 25 small amount of incomplete combustion (e.g., exhaust particulates). In a liquid 26 hydrocarbon fueled engine, these incombustibles provide a small but critical 27 degree of lubrication to the exhaust valve/seat interface, thereby ensuring the 28 durability of both cylinder heads and valves. The combustion of natural gas 29 fuel is often very complete, with virtually no incombustible materials. 30 Therefore, the durability of the cylinder head and valve is controlled by the 31 ash content and other properties of the lubricating oil and its consumption 32 rate. There are no incombustible materials to aid in lubrication to the exhaust 33 valve/seat interface in a natural gas fueled engine. Natural gas fueled engines -16- 1 burn fuel that is introduced to the combustion chamber in the gaseous phase. 2 This has a significant affect on the intake and exhaust valves because there is 3 no fuel-derived lubricant for the valves like liquid droplets or soot. 4 Consequently, gas engines are solely dependent on the lubricant ash to 5 provide lubricant between the hot valve face and its mating seat. Too little ash 6 or the wrong type can accelerate valve and seat wear, while too much ash 7 may lead to valve guttering and subsequent valve torching. Too much ash can 8 also lead to detonation from combustion chamber deposits. Consequently, 9 gas engine builders frequently specify a narrow ash range that they have io learned provides the optimum performance. Since most gas is low in sulfur, 11 excess ash is generally not needed to address alkalinity requirements, and 12 ash levels are largely optimized around the needs of the valves. There may 13 be exceptions to this in cases where sour gas or landfill gas is used. 14 Natural gas fueled engine lubricating oils are classified according to 15 their ash content. The lubricant ash acts as a solid lubricant to protect the 16 valve/seat interface in place of naturally occurring exhaust particles in a 17 hydrocarbon fueled engine. The oil industry has accepted guidelines that s18 classify natural gas fueled engine lubricating oil according to their ash level. 19 The classifications of natural gas fueled engine lubricating oil according to 20 their ash levels are presented in Table 2. 21 22 TABLE 2 23 Classifications of Lubricating Oils for 24 Natural Gas Fueled Engines According To Ash Levels Ash Designation Sulfated Ash Level (wt. %. Determined by ASTM D874) Ashless 0 <Ash < 0.15 Low Ash 0.15 < Ash < 0.6 Medium Ash 0.6 < Ash < 1.0 High Ash Ash > 1.0 25 26 The ash level of lubricating oil is often determined by its formulation 27 components. Metal-containing detergents (e.g., barium, calcium) and 28 metallic-containing wear inhibitors contribute to the ash level of lubricating 29 oils. For correct engine operation, gas engine manufacturers define lubricating 30 oil ash requirements as part of the lubricating oil specifications. For example, -17- 1 manufacturers of 2-cycle engines often require natural gas engine lubricating 2 oil to be Ashless to minimize the extent of harmful deposits that form on the 3 piston and combustion chamber area. Manufacturers of 4-cycle engines often 4 require natural gas engine lubricating oils to be Low, Medium or High Ash 5 levels, refer to Table 2, to provide the correct balance of engine cleanliness 6 and durability of the cylinder head and valves. Running the engine with 7 lubricating oil with too low an ash level will likely result in shortened life for the 8 valves or cylinder head. Running the engine with lubricating oil having too 9 high an ash level will likely cause excessive deposits in the combustion o10 chamber and upper piston area. 11 The degree of nitration of the lubricating oil may vary significantly 12 depending on the engine design and operating conditions. Lean burn engines 13 produce less NOx than their stoichiometric counterparts, so they tend to 14 nitrate the oils less. Some operators may richen the air/fuel mixture on natural 15 gas fueled engines to increase power output and consequently increase oil 16 nitration levels. Lubricating oils with good nitration resistance are required in 17 most natural gas engine installations because the lubricating oil may be used 18 to lubricate a number of engines including stoichiometric and lean-burn 19 models. 20 This invention will be further illustrated by the following examples that set 21 forth particularly preferred embodiments. While the examples are provided to 22 illustrate this invention, they are not intended to limit it. 23 24 EXAMPLES 25 The Examples describe experiments performed using Samples A 26 through C. Multiple experiments were performed in each example using a 27 variety of detergents such as, but not limited to, sulfonate, phenate and 28 salicylate detergents; succinimide dispersants; and zinc dithiophosphate wear 29 inhibitors. The examples are explained using the terms detergent, dispersant 30 and wear inhibitor because no significant difference was found when these 31 components were varied. 32 Sample A was prepared by combining about 2 wt. % 2-(4-hydroxy-3, 33 5-di-t-butyl benzyl thiol) acetate, about 3 wt. % dispersant, about -18- 1 3.4 wt. % detergent, about 0.38 wt. % wear inhibitor, 5 ppm foam inhibitor and 2 Group II base oil. Sample A was prepared by combining the components at 3 140 degrees Farenheit (F) ( 60 degrees Celsius) with agitation until all 4 components were mixed. 5 Sample B was prepared by combining about 2 wt. % 2-(4-hydroxy-3, 6 5-di-t-butyl benzyl thiol) acetate, about 3 wt. % dispersant, about 7 3.4 wt. % detergent, about 0.38 wt. % wear inhibitor, 5 ppm foam inhibitor and 8 Group I base oil. Sample B was prepared by combining the components at 9 140 degrees F (60 degrees Celsius) with agitation until all components were 10 mixed. 11 Sample C was prepared by using OLOA 1255, commercially available 12 from Chevron Oronite Company in Houston, Texas. The OLOA 1255 was 13 mixed with Group I base oil under typical blending conditions of about 14 140 degrees F (60 degrees Celsius) with agitation until all components were 15 thoroughly mixed. As explained in of U.S. Pat. No. 5,726,133 lubricating oil 16 made by combining OLOA 1255 is one of the most widely sold gas engine oil 17 additive formulations and represents, therefore, a "benchmarkstandard" 18 against which other formulations useful as engine oils may be measured. 19 20 EXAMPLE1 21 The Oxidation-Nitration And 22 Viscosity Increase Resistance Test 23 24 The Oxidation-Nitration and Viscosity Increase Resistance bench test 25 demonstrates the capacity of lubricating oil to resist oxidation, nitration and 26 viscosity increase. This test is a tool to help determine the performance of oils 27 as they relate to the actual service of lubricating engines that use natural gas as 28 a fuel source. The level of oxidation and nitration of oil, may also be compared 29 by monitoring the viscosity increase of the oil. The lower the values for 30 oxidation, nitration and viscosity increase at the end the test, the more superior 31 the product's performance. The Oxidation-Nitration and Viscosity Increase 32 Resistance bench test was designed to simulate Caterpillar 3500 series engine 33 conditions as related to actual field performance of the Caterpillar 3516 model. 34 Oxidation-Nitration and Viscosity Increase Resistance tests were performed on - 19- 1 Samples A, B, C, and D. The samples were placed in a heated glassware bath 2 and subjected to calibrated levels of nitrous oxide gas over a specific period of 3 time. The tests were run on each sample in duplicate and the results are an 4 average of the two runs. The samples were evaluated using differential infra 5 red spectroscopy before placing them in the heated glassware bath to 6 determine a base line for each sample. The samples were re-evaluated at the 7 end of testing period. The differential between the base line data, absorbance 8 units at 5.8 and 6.1 microns, and the data taken at the end of test cycle 9 provides an indication of the oxidation-nitration resistance of the samples. 10 Differential infra red spectroscopy measures the amount of light that is 11 absorbed by an oil sample and provides a unit of measure called an 12 absorbance unit. DIR (Differential Infrared) spectra was determined by 13 subtracting the fresh oil spectra from the used oil spectra to observe changes 14 that have occurred due to oxidation, nitration, fuel dilution, soot accumulation, 15 and or contamination. Typically a 0.1 millimeter (mm) cell is used, however an 16 ATR crystal setup may be used after determining its associated path length. If 17 the instrument does not have software that determines path length, the path 18 length may be back calculated by measuring oxidation with a calibrated 19 0.1 mm cell. The variation between ATR and vertical cell measurements is 20 minimal if restricted to the narrow area of oxidation and nitration 21 (~1725 to 1630 cm-1). 22 DIR Oxidation was measured from peak maximum at ~1715 ± 5 cm 1 to 23 the spectra baseline (in units of absorbance). 24 DIR Nitration was measured from peak maximum at -1630 ± 1 cm " 1 to 25 peak baseline (in units of absorbance). 26 - 20 - 1 GRAPH I 2 Infrared Spectra 0626 0.24 0,22 0.22 0.20 Nitration is measured pak max to peak baseline at 1630 cm-1 0.1 Oxidation is measured 014eak max to snec baseline at ~1715 cm-1 0.12 0.06 0.04 N 0.02 2000 1900 1800 1700 1600 1500 Wavenumbers (cm-1) 3 4 Oxidation (&/or Nitration) Number Reported (abs/cm)= peak absorbance divided by path length in cm- (report in 5 whole numbers) 6 7 During the Oxidation-Resistance Bench Test, the viscosity increases of 8 the samples were measured at 100C by ASTM D 445. The viscosity increase 9 is a percentage that compares the initial '"fresh" kinematic viscosity with the end 10 of test "used" oil kinematic viscosity. The formula to calculate for % viscosity 11 difference is: 12 13 % Viscosity difference = (Sample (x) initij - Sample (x) fnal)/Sample (x) initial x 100 % 14 15 Oxidation levels of 5.8 microns and Nitration levels of 6.1 microns were 16 used as peak height comparisons. 17 Measurements are reported on a relative measurement basis so that 18 large results or values represent greater levels of oxidation-nitration and 19 viscosity increase resistance. Lower numbers represent shorter oil life. 20 Sample C was used as a reference oil and the results in the Tables 3, 4 and 21 5 were reported as a ratio in the first row of each table. This ratio was 22 calculated by dividing measurements for Sample C by the measurements taken 23 using the sample being compared to sample C. The second row of each table -21 - 1 displays the percent difference between the reference Sample C and the 2 samples being compared to Sample C. The larger the percentage difference 3 between Sample C and the other samples, the better performing the sample in 4 respect to oxidation resistance. Sample C was the reference sample for the 5 results reported in Table 3-5. The formula to calculate percentage difference of 6 the ratios compared to Sample C for Tables 3-5 is: 7 8 % difference = (Sample (x) - Sample C)/Sample (x) x 100 % 9 10 TABLE 3 11 Oxidation Resistance Test Results Sample A Sample B Sample C Ratio* 3.8 1.5 1.0 % Difference compared to Sample D** 74 32 0 12 *Ratio - These numbers are relative ratios compared to Sample C's performance in this test. Numbers larger than 13 1.00 perform better than Sample C and less than 1.00 perform worse than the reference. The higher the ratio 14 number, the higher the performance of the sample. 15 16 **% Difference - These numbers are the percentage difference between Sample C and the comparative Sample. A 17 negative number indicates worse performance than Sample C. 18 19 The results presented in Table 3 indicate that Sample A and B exhibited a 20 74 % and 32 % improvement, respectively, in oxidative resistance over the 21 reference Sample C. 22 23 TABLE4 24 Nitration Resistance Test Results Sample A Sample B Sample C Ratio* 8.7 1.8 1.0 % Difference compared to Sample D** 89 44 0 25 26 *Ratio - These numbers are relative ratios compared to Sample C's performance in this test. Numbers larger than 27 1.00 perform better than Sample C and less than 1.00 perform worse than the reference. The higher the ratio 28 number, the higher the performance of the sample. 29 30 **% Difference - These numbers are the percentage difference between Sample C and the comparative Sample. A 31 negative number indicates worse performance than Sample C. 32 33 The results in Table 4 indicate the improved performance of Samples A 34 and B over the reference sample. The improvement ranged from 35 89 % to 44 % over the reference sample in nitration resistance. 36 -22 - 1 TABLE 5 2 Viscosity Increase Resistance Test Results Sample A Sample B Sample C Ratio* 7.0 3.0 1.0 % Difference compared to Sample 86 67 0 C** 3 *Ratio - These numbers are relative ratios compared to Sample C's performance in this test. Numbers larger than 4 1.00 perform better than Sample C and less than 1.00 perform worse than the reference. The higher the ratio 5 number, the higher the performance of the sample. 6 7 **% Difference - These numbers are the percentage difference between Sample C and the comparative Sample. A 8 negative number indicates worse performance than Sample C. 9 10 The results in Table 5 indicate the improved performance of Samples A 11 and B over the reference sample. The improvement ranged from 86 % to 12 67 % over the reference sample in viscosity increase resistance. 13 Samples A and B perform superior to the reference sample with 14 respect to oxidation, nitration and viscosity increase. These tests that quantify 15 a lubricating oil's resistance to oxidation and nitration and the resultant 16 viscosity increase are used to determine whether samples are good 17 candidates for extending the life of lubricating oil particularly those lubricating 18 oils for use in natural gas fueled engines. Absorbing oxygen and nitrogen and 19 the resultant viscosity increase associated with absorbing oxygen and 20 nitrogen are undesirable for lubricating oil particularly lubricating oils for use in 21 natural gas fueled engines. 22 - 23 -

Claims (30)

WHAT IS CLAIMED IS:

1. Lubricating oil comprising a minor amount of one or more hindered phenols of the general formula:

o

wherein R is a C -Cg alkyl group and a major amount of at least one of Group 11, 111 and IV base oils.

2 Lubricating oil of Claim 1 wherein about 0.2 wt. % to about 3 wt. % of the lubricating oil comprises the hindered phenol of Claim 1.

3. Lubricating oil of Claim 1 wherein the hindered phenol is liquid.

4 A additive formulation comprising the hindered phenol of Claim 1 and one or more dispersants, one or more detergents and one or more wear inhibitors.

5 Lubricating oil of Claim 1 having a total base number of about 2.15 to about 8.88 as determined by ASTM D 2896.

6 Lubricating oil of Claim 1 having a total ash content of about 0.1 wt. % to about 1.5 wt. % as determined by ASTM D874.

7 Lubricating oil of Claim 1 having less than 4000 ppm sulfur.

8 A method of making the lubricating oil of Claim 1 by combining the hindered phenol of Claim 1 with the base oil of Claim 1 in any order and mixing.

9. A method of lubricating engines comprising lubricating one or more engines with lubricating oil comprising the hindered phenol of Claim 1.

10. Lubricating oil comprising a major amount of at least one of Group II, III and IV base oil and a minor amount of 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate.

11. Lubricating oil of Claim 10 having less than 4000 ppm sulfur.

12. Lubricating oil of Claim 10, wherein about 0.2 wt. % to about 3 wt. % of the lubricating oil comprises 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate.

13. Lubricating oil of Claim 10, wherein the 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate is liquid.

14. Lubricating oil of Claim 10 having a total base number of about 2.15 to about 8.88 as determined by ASTM D 2896.

15. Lubricating oil of Claim 10 having a total ash content of about 0.1 wt. % to about 1.5 wt. % as determined by ASTM D874.

16. A method of lubricating engines comprising contacting the engine with the lubricating oil of Claim 10.

17. A method for making the lubricating oil of Claim 10 comprising combining the 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate with at least one of Group II, III and IV base oil in any order and mixing.

18. An additive formulation comprising 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate; one or more dispersants; one or more wear inhibitors; and at least one of sulfonate, salicylate and phenate detergents.

19. Lubricating oil comprising a minor amount of the additive formulation of Claim 18 and a major amount of at least one of Group II, III and IV base oil.

20. Lubricating oil of Claim 19 comprising about 4 wt. % to about 16.50 wt. % of the additive formulation and about 40 wt. % to about 97 wt. % of the base oil.

21. A method for lubricating a natural gas fueled engine comprising lubricating the natural gas fueled engine with the lubricating oil of Claim 19.

22. A method for making the additive formulation of Claim 18, comprising combining one or more detergents; one or more dispersants; one or more wear inhibitors; and 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate in any order and agitating until mixed.

23. Lubricating oil comprising:

about 1 wt. % to about 8 wt. % of one or more dispersants;

about 1 wt. % to about 8.5 wt. % of one or more detergents;

about 0.2 wt. % to about 1.5 wt. % of one or more wear inhibitors;

about 0.2 wt. % to about 3 wt. % 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate; and

about 6 wt. % to about 97 wt. % of at least one of Group II, III and IV base oil.

24. A method for lubricating a natural gas fueled engine comprising lubricating the natural gas fueled engine with the lubricating oil of Claim 23.

25. Lubricating oil comprising:

about 1 wt. % to about 8 wt. % of one or more dispersants;

about 1 wt. % to about 8.5 wt. % of one or more detergents;

about 0.2 wt. % to about 1.5 wt. % of one or more wear inhibitors;

about 0.6 wt. % to about 2.5 wt. % 2-(4-hydroxy-3, 5-di-t-butyl benzyl thiol) acetate; and

about 6 wt. % to about 97 wt. % of at least one of Group II, III and IV base oil.

26. A method of lubricating engines comprising contacting one or more engines with the lubricating oil of Claim 25.

27. A method of reducing oxidation in an internal combustion engine which comprises operating an engine with a composition comprising

a minor amount of one or more hindered phenols of the general formula: o

wherein R is a C₇-C₉ alkyl group and a major amount of at least one of Group 11, 111 and IV base oils.

28. A method of reducing oxidation in an internal combustion engine according to claim 27 wherein said internal combustion engine is a natural gas engine.

29. The use of an antioxidant system comprising

a minor amount of one or more hindered phenols of the general formula: o

wherein R is a C₇-C₉ alkyl group and a major amount of at least one of Group II, III and IV base oils, to reduce oxidation in an internal combustion engine.

30. The use of an antioxidant system according to claim 29 wherein said internal combustion engine is a natural gas engine.