EP1266955B1

EP1266955B1 - Lubricating oil compositions

Info

Publication number: EP1266955B1
Application number: EP02076993.1A
Authority: EP
Inventors: Ian Alexander Weston Bell; Robert Robson
Original assignee: Infineum International Ltd
Current assignee: Infineum International Ltd
Priority date: 2001-06-15
Filing date: 2002-05-23
Publication date: 2013-11-06
Anticipated expiration: 2022-05-23
Also published as: EP1266955A1

Description

This invention relates to lubricating oil compositions such as multigrade lubricants that, in particular, give enhanced performance in diesel engine piston cleanliness and piston ring-sticking tests.
Lubricating oil compositions (or lubricants) for the crankcase of internal combustion engines are well-known and it is also well-known for them to contain additives (or additive components) to enhance their properties and performance.
Increasingly, the demands of original equipment manufacturers (OEM's) to meet performance criteria dictate the properties of lubricants. One such performance criterion concerns the sticking of piston rings during operation of a compression-ignited (diesel) internal combustion engine. This is usually referred to briefly as "ring-sticking"; it may be measured by the VWTDi test (CEC L-78-T-99). A second performance criterion that is measured in this test is piston cleanliness.
Other performance criteria of interest include the volatility of the lubricant, the fuel economy performance of the lubricant, and the chlorine content of the lubricant.
The various criteria clearly constrain formulators of lubricants in terms of additive components and amounts, and of basestocks, that may be used.
EP-A-1 087 008 describes a way of meeting "ring-sticking" test requirements by provision of certain additive components.
WO 99/18175 discloses a lubricating oil composition comprising a synthetic ester which maintains particulate combustion products in suspension.
EP 0,931,827 discloses a lubricating oil composition comprising an organomolybdenum compound, ZDDP and an ester blended base stock.
The present invention provides a different approach, i.e. by providing a specific basestock mixture in order to meet the above mentioned requirements.
US-A-4 968 452 discloses a very similar lubricating oil composition. The purpose of the oil is the same as in US-A-4 960 542 , but the composition differs in respect of the requirements of the characteristics of the mineral oil. Specifically, the mineral oil component has a higher pour point temperature, a maximum aromatics content of 3% by weight, and a viscosity index of at least 80, preferably at least 85.
EP-A-0 931 827 relates to lubricating oils for internal combustion engines in which the oil content of the composition comprises from 70 to 90 weight % of a poly (alpha-olefin) and/or a highly refined mineral oil, and 30 to 10 % by weight of diesters and polyol esters. The mineral oil is described in terms of requiring a specific kinematic viscosity range, a sulphur content of 5 ppm or lower and an aromatics content of not more than 1 wt %. The viscosity index required in the oil is not disclosed.
EP-A-0 552 554 relates to a lubricating oil composition having a major proportion of a biodegradable base fluid which is a blend of an ester of isotridecyl alcohol and a mono, di or polycarboxylic acid and an aliphatic hydrocarbon oil containing no more than 10 % by weight of aromatic hydrocarbons. The ester component of this composition is not a polyol ester. The hydrocarbon oil described does not meet the requirements of a Group III basestock
EP-A-1 087 008 describes a way of meeting "ring-sticking" test requirements by provision of certain additive components.
The present invention provides a different approach, i.e. by providing a specific basestock mixture in order to meet the above mentioned requirements.
In a first aspect, the invention is a crankcase lubricating oil composition, preferably for a compression-ignition engine, especially for a passenger car compression-ignition engine, comprising, or made by admixing, a major amount of providing in excess of 50 mass% of the composition, of :

(A) a basestock of lubricating viscosity consisting essentially of a Group III basestock as both defined in American Petroleum Institute 1509 "Engine oil Licensing and Certification System" Fourteenth Edition, December 1996, the Group II basestock being in the form of a polyolester, the polyolester being present in an amount of 1 to 8 mass %, based on the mass of the basestock; and a Group V basestock, and minor amounts of lubricant additive components, providing less than 50 mass % of the composition, comprising
(B) a dispersant, such as an ashless dispersant;
(C) a metal detergent;
(D) one or more other lubricant additive components selected from anti-oxidants, anti-wear agent and friction modifiers; and
(E) a viscosity modifier.

The data contained in the specification demonstrate that the use of the basestock mixture (A) unexpectedly improves the performance of lubricating oil compositions in the TDi test.
In a second aspect, the invention is a method of lubricating a compression-ignited internal combustion engine comprising operating the engine and lubricating the engine with a lubricating oil composition according to the first aspect of the invention.
In a third aspect, the invention is a method of reducing the ring-sticking tendencies and improving the piston cleanliness of a compression-ignited internal combustion engine comprising adding to the engine a lubricating oil composition according to the first aspect of the invention.
In a fifth aspect, the invention is the use of a lubricating oil composition according to the first aspect to reduce the ring-sticking tendencies and to improve the piston cleanliness of a compression-ignited internal combustion engine.
In this specification:

the term "comprising" or "comprises" when used is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof;
the term "consisting essentially of" or its cognate when used is taken to specify the presence of stated features, integers, steps or components but may include the presence or addition of one or more other features, integers, steps, components or groups thereof, provided their inclusion does not substantially affect the invention;
the term "consisting of" or its cognate when used is taken to specify the presence of stated features, integers, steps or components but precludes the presence or addition of one or more other features, integers, steps, components or groups thereof; accordingly, whenever the term "comprising" or its cognate is used, a preferred embodiment is deemed the term "consisting essentially of" or its cognate, and consequently, the term "consisting of" or its cognate is deemed a preferred embodiment of the term "consisting essentially of" or its cognate;
"major amount" means in excess of 50 mass % of the composition;
"minor amount" means less than 50 mass % of the composition both in respect of the stated additive and in respect of the total mass % of all of the additives present in the composition, reckoned as active ingredient of the additive or additives;
"oil-soluble" or "oil-dispersible" used herein do not necessarily indicate that the compounds or additives are soluble, dissolvable, miscible, or capable of being suspended in the oil in all proportions. These do mean, however, that they are, for instance, soluble or stably dispersible in oil to an extent sufficient to exert their intended effect in the environment in which the oil is employed. Moreover, the additional incorporation of other additives may also permit incorporation of higher levels of a particular additive, if desired;
the acronym SAE stands for Society of Automotive Engineers; and
all percentages reported are mass % on an active ingredient basis, i.e., without regard to carrier or diluent oil, unless otherwise stated.

The invention also provides the product obtained or obtainable as a result of any reaction between the various additive components of the composition or concentrates, essential as well as customary and optimal, under the conditions of formulation, storage or use.
The features of the invention will now be discussed in more detail as follows:

Multigrade lubricants

Multiviscosity grade oils, commonly referred to as "multigrades" are designed to operate over wide temperature ranges and are identified by descriptors such as SAE 10W-30 or SAE 5W-30. Their properties are defined in the Society of Automotive Engineers document SAE J300. This publication defines multigrades in terms of two criteria:

Maximum low temperature cranking and pumping viscosities and
Maximum and minimum kinematic viscosities at 100°C and a minimum high-shear viscosity at 150°C and 10⁸s^-1.

Low temperature properties define which "W" grade shall be assigned to a lubricant and high temperature properties define the "non W" part of the designation.
SAE J300 defines a series of W grades with SAE 0W representing the requirements for operation at lowest temperatures. SAE 5W, 10W, 15W, 20W and 25W are also defined; these grades are suitable for progressively higher minimum temperatures of operation.
Non-W grades are also assigned a numerical designation, which corresponds to a scale of high temperature viscosity. This scale starts with SAE 20 and goes through SAE 30, 40 and 50 to the most viscous grade, SAE 60.
This system of viscometric classification of automotive crankcase lubricants finds universal application with the vehicle and lubricant manufacturing industries.
In a preferred embodiment, the lubricating oil composition of the present invention is a multigrade lubricating oil composition in the form of an SAE 15W-X, SAE 10W-X, SAE 5W-X or 0W-X composition, wherein X represents any one of 20, 30 and 40. Preferably, the oil composition is in the form of an SAE 5W-X or 0W-X. Advantageously, X represents either 20 or 30, especially 20.

(A) Basestock

The basestock (sometimes referred to as "base oil") is an oil of lubricating viscosity and is the primary liquid constituent of a lubricant into which additives and possibly other oils are blended to produce the final lubricant.
American Petroleum Institute (API) 1509 "Engine Oil Licensing and Certification System" Fourteenth Edition, December 1996 states that all basestocks are divided into five general categories:

a) Group I basestocks contain less than 90 per cent saturates and/or greater than 0.03 per cent sulfur and have a viscosity index greater than or equal to 80 and less than 120;
b) Group II basestocks contain greater than or equal to 90 per cent saturates and less than or equal to 0.03 per cent sulfur and have a viscosity index greater than or equal to 80 and less than 120;
c) Group III basestocks contain greater than or equal to 90 per cent saturates and less than or equal to 0.03 per cent sulfur and have a viscosity index greater than or equal to 120;
d) Group IV basestocks are polyalphaolefins (PAO);
e) Group V basestocks include all other basestocks not included in Group I, II, III, or IV.

The test methods used in defining the above groups are ASTM D2007 for saturates; ASTM D2270 for viscosity index; and one of ASTM D2622, 4294, 4927 and 3120 for sulfur.
As stated, the basestock (A) in the present invention consists essentially of a Group III basestock and a Group V basestock in the form of an ester. The amount of Group V basestock in the form of an ester is present in an amount of 1 to 8, advantageously 3 to 8, such as 5 to 8, mass %, based on the mass of the total basestock.
Basestock (A) consists essentially of Group III basestocks and Group V basestocks in the form of an ester, but may contain minor amounts, such as at most 25, such as at most 20, preferably at most 10, advantageously at most 5, mass %, based on the mass of the total basestock, of other basestocks, such as Group I, Group II or Group IV basestocks or any mixture thereof. The Group I, Group II or Group IV basestocks or any mixture thereof may be used as a diluent or carrier fluid for the additive components used in preparing the oil compositions of the invention.
Group III basestocks are commercially available.
Group V basestocks in the form of polyolesters are also commercially available. Examples include pentaerythritol esters, trimethylol propane esters and neopentylglycol esters; diesters; C₃₆ dimer acid esters; trimellitate esters, i.e. 1, 2, 4-benzene tricarboxylates; and phthalate esters, i.e. 1,2-benzene dicarboxylates. The acids from which the esters are made are preferably monocarboxylic acids of the formula RCO₂H where R represents a branched, linear or mixed alkyl group. Such acids may, for example, contain 6 to 18 carbon atoms. A preferred polyol ester is an ester of trimethylol propane with a mono-basic acid.

(B) Dispersants

Dispersants, such as ashless dispersants, i.e. non-metallic organic materials that form substantially no ash on combustion, hold solid and liquid contaminants in suspension and comprise long-chain hydrocarbons, to confer oil-solubility, with a polar head capable of associating with particles to be dispersed. A noteworthy group is hydrocarbon-substituted succinimides.
"Substantially no ash" means that the dispersant may give trace amounts of ash on combustion, but amounts which do not have practical or significant effect on the performance of the dispersant.

(C) Metal detergents

A detergent is an additive that reduces formation of piston deposits, for example high temperature varnish and lacquer deposits, in engines; it normally has acid-neutralising properties and is capable of keeping finely-divided solids in suspension. Most detergents are based on metal "soaps", that is metal surfactants or salts of acidic organic compounds. Examples of organic acids include sulfonic acids, phenols and sulfurised derivatives thereof, and carboxylic acids including aromatic carboxylic acids, such as salicylic acids.
Detergents generally comprise a polar head with a long hydrophobic tail, the polar head comprising a metal salt of an acidic organic compound. The salts may contain a substantially stoichiometric amount of the metal, in which case they are usually described as normal or neutral salts, and would typically have a total base number or TBN (as may be measured by ASTM D2896) of from 0 to 80. Large amounts of a metal base can be included by reacting an excess of a metal compound, such as an oxide or hydroxide, with an acidic gas such as carbon dioxide. The resulting overbased detergent comprises neutralised detergent as the outer layer of a metal base (e.g. carbonate) micelle. Such overbased detergents may have a TBN of 150 or greater, and typically of from 250 to 450 or more.
The detergents that may be used include oil-soluble neutral and overbased sulfonates, phenates, sulfurized phenates, thiophosphonates, salicylates, and naphthenates. Particularly convenient detergents are neutral and overbased calcium sulfonates having a TBN of from 20 to 450 TBN, and neutral and overbased calcium phenates and sulfurized phenates having a TBN of from 50 to 450.
The detergents of the present invention may be salts of one type of organic acid or salts of more than one type of organic acids, for example hybrid complex detergents.
In an embodiment, the detergent comprises metal salts of one type of organic acid.
A hybrid complex detergent is a detergent in which the basic material within the detergent is stabilised by metal salts of more than one type of organic acid. It will be appreciated by one skilled in the art that a single type of organic acid may contain a mixture of organic acids of the same type. For example, a sulfonic acid may contain a mixture of sulfonic acids of varying molecular weights. Such an organic acid composition is considered as one type. Thus, complex detergents are distinguished from mixtures of two or more separate, optionally overbased, detergents, an example of such a mixture being one of an overbased calcium salicylate detergent with an overbased calcium phenate detergent.
The art describes examples of overbased complex detergents. For example, International Patent Application Publication Nos. 9746643/4/5/6 and 7, which are incorporated herein in respect of the description and definition of the hybrid complex detergents, describe hybrid complexes made by neutralising a mixture of more than one acidic organic compound with a basic metal compound, and then overbasing the mixture. Individual basic micelles of the detergent are thus stabilised by a plurality of organic acid types. Examples of hybrid complex detergents include calcium phenate-salicylate-sulfonate detergents, calcium phenate-sulfonate detergents and calcium phenate-salicylate detergents.
EP-A-0 750 659 describes a calcium salicylate phenate complex made by carboxylating a calcium phenate and then sulfurising and overbasing the mixture of calcium salicylate and calcium phenate. Such complexes may be referred to as "phenalates"
Preferred complex detergents are salicylate-based detergents, for example, a calcium phenate-salicylate detergent and "phenalates".

(D) Other lubricant additives

(i) anti-oxidants increase the composition's resistance to oxidation and may work by combining with and modifying peroxides to render them harmless by decomposing peroxides or by rendering an oxidation catalyst inert. They may be classified as radical scavengers (e.g., sterically hindered phenols, secondary aromatic amines, and organocopper salts); hydroperoxide decomposers (e.g., organo-sulfur and organophosphorus additives); and multifunctionals. In the practice of the present invention, the use or otherwise of certain anti-oxidants may confer certain benefits. For example, in one embodiment it may be preferred that the lubricating oil composition is free of any secondary aromatic amine anti-oxidants. In another embodiment, it may be preferred to employ in the lubricating oil composition a combination of one or more secondary aromatic amine anti-oxidants (eg in the range of 0.1 to 0.7, preferably 0.2 to 0.5, mass % of the composition) and one or more sterically hindered phenol anti-oxidants (e.g., in the range of 0.1 to 2, preferably 0.5 to 1.5, mass % of the composition); such composition may for example contain one or more molybdenum-containing additives in an amount providing from 50 or 100 to 500 or 700 ppm by mass of elemental molybdenum in the composition.
(ii) anti-wear agents reduce friction and excessible wear and are usually based on compounds containing sulfur or phosphorus or both. Noteworthy are metal dihydrocarbyl dithiophosphates such as zinc dialkyl dithiophosphates (ZDDP's). Preferably, the alkyl groups are essentially secondary alkyl groups.
(iii) friction modifiers include boundary additives that lower friction coefficients and hence improve fuel economy. Examples are esters of polyhydric alcohols such as glycerol monoesters of higher fatty acids, for example glycerol mono-oleate; esters of long chain polycarboxylic acids with diols, for example the butane diol esters of dimerized unsaturated fatty acids; oxazoline compounds; and alkoxylated alkylsubstituted mono-amines, and alkyl ether amines, for example, ethoxylated tallow amine and ethoxylated tallow ether amine. Preferably, in the practice of this invention, component(s) (D) includes one or more friction modifiers selected from esters of polyhydric alcohols and from alkoxylated amines.

Molybdenum compounds, such as dinuclear and trinuclear dithiocarbamates and dithiophosphates, are also examples of friction modifiers

(E) Viscosity Modifiers

Viscosity modifiers (or viscosity index improvers) impart high and low temperature operability to a lubricating oil. Viscosity modifiers that also function as dispersants are also known and may be prepared as described above for ashless dispersants. In general, these so-called dispersant viscosity modifiers are functionalized polymers (e.g., interpolymers of ethylene-propylene post-grafted with an active monomer such as maleic anhydride) which are then derivatized with, for example, an alcohol or amine.
Suitable compounds for use as viscosity modifiers are generally high molecular weight hydrocarbon polymers, including polyesters. Oil-soluble viscosity modifying polymers generally have weight average molecular weights of from 10,000 to 1,000,000, preferably 20,000 to 500,000, which may be determined by gel permeation chromatography or by light scattering.
Representative examples of suitable viscosity modifiers are polyisobutylene, copolymers of ethylene and propylene and higher alpha-olefins, polymethacrylates, polyalkylmethacrylates, methacrylate copolymers, copolymers of an unsaturated dicarboxylic acid and a vinyl compound, interpolymers of styrene and acrylic esters, and partially hydrogenated copolymers of styrene, isoprene, styrene/butadiene, and isoprene/butadiene, as well as the partially hydrogenated homopolymers of butadiene and isoprene and isoprene/divinylbenzene.
Other known additives may be incorporated into the lubricating oil compositions of the invention, being different from those defined in the invention. They may, for example, include other detergents, rust inhibitors, corrosion inhibitors, pour point depressants, anti-foaming agents, and surfactants. They can be combined in proportions known in the art.
As is known in the art, some additives can provide a multiplicity of effects; thus, for example, a single additive may act both as a dispersant and as an anti-oxidant.

CONCENTRATES

In the preparation of lubricating oil compositions, it is common practice to introduce additive(s) therefor in the form of concentrates of the additive(s) in a suitable oleaginous, typically hydrocarbon, carrier fluid, e.g. mineral lubricating oil, or other suitable solvent. Oils of lubricating viscosity such as described above, as well as aliphatic, naphthenic, and aromatic hydrocarbons, are examples of suitable carrier fluids for concentrates.
Concentrates constitute a convenient means of handling additives before their use, as well as facilitating solution or dispersion of additives in lubricating oil compositions. When preparing a lubricating oil composition that contains more than one type of additive, each additive may be incorporated separately - each in the form of a concentrate. In many instances, however, it is convenient to provide a so-called additive "package" (also referred to as an "adpack") comprising two or more additives in a single concentrate.
A concentrate may contain 1 to 90, such as 10 to 80, preferably 20 to 80, more preferably 20 to 70, mass % active ingredient of the additive or additives.

MAKING OIL COMPOSITIONS

Lubricating oil compositions may be prepared by adding to an oil of lubricating viscosity a mixture of an effective minor amount of at least one additive and, if necessary, one or more co-additives such as described herein. This preparation may be accomplished by adding the additive directly to the oil or by adding it in the form of a concentrate thereof to disperse or dissolve the additive. Additives may be added to the oil by any method known to those skilled in the art, either prior to, contemporaneously with, or subsequent to addition of other additives.
Typically, lubricating oil compositions are prepared by adding a concentrate comprising detergent inhibitor additives, such as anti-wear, dispersant, anti-oxidant, detergent and friction modifying additives, and a separate concentrate comprising a viscosity modifier additive, to a basestock.
Therefore, one or more additive components and one or more concentrates may also contribute basestock, which may be a Group I, Group II or Group IV basestock or any mixture thereof, to a lubricating oil composition. Thus, the basestock (A) means the total basestock that is present in the lubricating oil composition.
The lubricating oil compositions may be used to lubricate mechanical engine components, particularly an internal combustion, such as a compression-ignited, engine, by adding the lubricating oil thereto. Particular examples of compression-ignited engines are those developed in recent years where the top ring groove temperature may exceed 150, preferably exceed 250, °C, due to increases in specific power output to around 5 or greater, such as 25 or greater, preferably at least 30, especially 40 or greater, kW/litre. Preferably the maximum specific power output is around 60 kW/litre. These engines are more prone to suffer from ring-sticking problems in their operation.

When lubricating oil compositions contain one or more additives, each additive is typically blended into the base oil in an amount which enables the additive to provide its desired function. Representative effective amounts of such additives, on an active ingredient basis, when used in crankcase lubricants, are listed below:

ADDITIVE	mass % (broad)	mass % (preferred)
Ashless Dispersant	0.1 - 20	1 - 8
Metal detergents	0.1 - 6	0.2 - 4
Corrosion Inhibitor	0 - 5	0 - 1.5
Metal dihydrocarbyl dithiophosphate	0.1 - 6	0.1 - 4
Supplemental anti-oxidant	0 - 5	0.01 - 1.5
Pour Point Depressant	0.01 - 5	0.01 - 1.5
Anti-Foaming Agent	0 - 5	0.001 - 0.15
Supplemental Anti-wear Agents	0 - 0.5	0 - 0.2
Friction Modifier	0 - 5	0 - 1.5
Viscosity Modifier	0 - 6	0.01 - 4
Mineral or Synthetic Base Oil	Balance	Balance

The final lubricating oil composition may contain from 5 to 25, preferably 5 to 18, typically 10 to 15, mass % of the concentrate (including any carrier fluid), the remainder being oil of lubricating viscosity.

EXAMPLES

The invention will now be particularly described, by way of example only, as follows:-

Preparation of lubricating Oil Compositions

Two lubricating oil compositions (or oils) were prepared, by methods known in the art, by blending an additive package, a basestock mixture, and a viscosity modifier. In the two oils, the additive package and the viscosity modifiers were the same and were present in the same proportions, expressed as mass %. The basestock mixtures were different such that one of the oils (Oil A) was a comparision oil, and the other oil (Oil 1) was an oil of the invention.

The formulations of Oils A and 1 are set out below where all figures represent mass % based on the total mass of the oil:

	Oil A	Oil 1
Additive Package	13.8	13.8
Viscosity Modifier concentrate	3.9	3.9
Group III basestock (commercially available)	77.1	72.3
Group V basestock (a trimethyol propane (TMP) ester with C8-C10 alkyl chains)	-	10.0
Group 1 basestock	5.2	-

Tests and Results

Samples of each of Oils A and 1 were subjected to an engine test used to investigate deposit formation, based specifically on the VWTDi CEC-L-78-T-99 test, also known as the PV1452 test. The test is regarded as an industry standard and as a severe assessment of a lubricant's performance capabilities.
The test employs a 4-cylinder, 1.9 litre, 81 kW passenger car diesel engine. It is a direct injection engine, in which a turbocharger system is used to increase the power output of the unit. The industry test procedure consists of a repeating cycle of hot and cold running conditions - the so-called PK cycle. This involves a 30 minute idle period at zero load followed by 180 minutes at full load and 4150 rpm. The entire cycle is then repeated for a total of 54 hours. In this 54 hour period the initial oil fill of 4.5 liters of test lubricant is not topped up.
At the end of the 54 hour test, the engine is drained, the engine disassembled and the pistons rated for piston deposits and piston ring sticking. This affords a result which is assessed relative to an industry reference oil (RL206) to define passing or failing performance.
The pistons are rated against what is known as the DIN rating system. The three piston-ring grooves and the two piston lands that lie between the grooves are rated on a merit scale for deposits and given a score out of 100 by a method known to those skilled in the art. In summary, the higher the number the better the performance: 100 indicates totally clean and 0 indicates totally covered with deposit. The five scores are then averaged to give the overall piston cleanliness merit rating. The scores for each of the four pistons are then averaged to afford the overall piston cleanliness for the test.
The rings are also assessed for ring sticking, which can occur due to excessive deposit build-up in the grooves. This is reported as an average over the rings on all the pistons, and also as the maximum ring sticking observed across the four pistons.
As indicated, these results are judged relative to an industry reference oil (RL206) to define passing performance.
In the test employed in this study, to obtain intermediate piston ratings, the engine was stopped every 12 hours, drained, stripped and rated, and re-assembled; the original test oil was put back into the engine which was then restarted.
The results obtained are tabulated below:

Average Piston Cleanliness (merits)

Inspection time (hours) 12 24 36 48 60

Oil A 69 65 63 - -

Oil 1 75 74 74 72 59
The results demonstrate that Oil 1 exhibits superior performance to Oil A.

Claims

A crankcase lubricating oil composition comprising, or made by admixing, a major amount, providing in excess of 50 mass% of the composition, of:
(A) a basestock of lubricating viscosity consisting essentially of a Group III basestock and a Group V basestock, as both defined in American Petroleum Institute 1509 "Engine oil Licensing and Certification System" Fourteenth Edition, December 1996, the group V basestock being in the form of a polyol ester, the polyol ester being present in an amount of 1 to 8 mass %, based on the total mass of the basestock;
and minor amounts of lubricant additive components, providing less than 50 mass% of the comporsition, comprising:
(B) a dispersant, such as an ashless dispersant;

(C) a metal detergent;

(D)one or more other lubricant additive components selected from anti-oxidants, anti-wear agent and fraction modifiers; and

(E) a viscosity modifier.
The composition as claimed in claim 1 wherein the polyol ester is present in an amount of 3 to 8 mass %, based on the mass of the total basestock.
The composition as claimed in claim 1 or claim 2 wherein the polyol ester is an ester of trimethylol propane with a mono-basic acid.
A method of lubricating a compression-ignited internal combustion engine comprising operating the engine and lubricating the engine with a lubricating oil composition as claimed in any of claims 1 to 3.
A method ofreducing the ring-sticking tendencies and improving piston cleanliness of a compression-ignited internal combustion engine comprising adding to the engine a lubricating oil composition as claimed in any of claims 1 to 3.
The use of a lubricating oil composition as claimed in any one of claims 1 to 3 to reduce the ring-sticking tendencies and to improve the piston cleanliness of a compression-ignited internal combustion engine.