CA2370468C

CA2370468C - Hydraulic fluid

Info

Publication number: CA2370468C
Application number: CA002370468A
Authority: CA
Inventors: Andrew John Holmes; Cameron William Watson
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1999-04-14
Filing date: 2000-04-11
Publication date: 2008-01-15
Anticipated expiration: 2020-04-11
Also published as: EP1173534B1; AU762520B2; JP2002542378A; CA2370468A1; ZA200108402B; KR100697579B1; AU3820300A; BR0009738A; CN1349556A; JP5378632B2; BR0009738B1; WO2000063325A1; CN1229477C; KR20020010602A; EP1173534A1

Abstract

A hydraulic fluid comprising a lubricant base oil in combination with (a) fr om 0.001 to 5 %wt of magnesium salicylate, (b) from 0.01 to 8 %wt of zinc dithiophosphate, and optionally (c) from 0.001 to 5 %wt of a dicarboxylic ac id or its mono- or di-ester or its mono-amide or di-amide or imide containing i n total between 4 and 70 carbon atoms; and an additive package comprising magnesium salicylate, zinc dithiophosphate and optionally a dicarboxylic aci d or its mono- or di-ester or its mono-amide or di-amide or imide containing i n total between 4 and 70 carbon.

Description

HYDRAULIC FLUID

The present invention relates to a hydraulic fluid.
Background of the Invention WO 93/03121 relates to hydraulic fluids having improved wet filterability and having increased resistance to loss of zinc when the oil is exposed to water. These compositions comprise (A) a combination of a metal salt selected from the group consisting of sulfonates, phenates, carboxylates and mixtures thereof, (B) an aliphatic carboxylic acid or anhydride, or carboxylic acid group containing derivative thereof, wherein the aliphatic group contains at least 20 carbon atoms, and optionally (C) a metal salt of (C)(I) at least one organic phosphorus acid or mixture of (C)(I) at least one organic phosphorus acid and (C)(II) at least one carboxylic acid group, and optionally (D) triazole. In one embodiment, the carboxylic acids of (A) are aromatic carboxylic acids. Zinc, calcium and magnesium salts of these aromatic carboxylic acids, and especially salicylic acids, are preferred. Although magnesium salicylate and zinc dithiophosphate are both mentioned per se, there is no teaching to use the combination of these compounds.
WO 89/04358 relates to compositions comprising (A) a neutral or basic metal salt of an acidic organic compound, (B) a metal deactivator, preferably an optionally substituted benzotriazole, and (C) a sulfur and/or phosphorus-containing compound selected from the group consisting of (C-1) phosphorus-containing amide, (C-2) phosphorus-containing ester, (C-3) sulfur-coupled dithiocarbamate, (C-4) sulfur-containing compound according to formula (I). Particularly preferred metals for the salts (A) are sodium, magnesium, calcium or mixtures of two or more thereof. In the extensive list of additional compounds which can be present, zinc dialkyl dithiophosphate is mentioned. Although magnesium salicylate and zinc dithiophosphate are both mentioned per se, there is no teaching to use the combination of these compounds.
EP-A-604 218 describes functional fluids comprising a major amount of an oil of lubricating viscosity, at least one metal salt of thiophosphoric acid, a zinc salt of salicylic acid, and a rust inhibitor, which fluid contains less than about 1 %wt of dispersant and less than about 1 %wt of basic sulfonate salt. The thiophosphoric acid is most commonly a dihydrocarbyl dithiophosphoric acid; the metal is most preferably zinc. The teaching of the document is restricted to the use of zinc salicylate.
In US-A-4,627,928, a process has been described for preparing basic magnesium salicylates which can be characterised as having a magnesium content of at least 150% up to 500%, of the stoichiometrically equivalent amount of magnesium based on the amount of total acid present. The magnesium salts obtained are described to be suitable for use as additives in general, more specifically as additives for a variety of lubricating oils and fuels, e.g. gasoline and diesel fuels. It is mentioned that these specific magnesium salicylates can be used in combination with a wide range of other compounds. There is no disclosure or teaching to apply a combination of zinc dithiophosphate and magnesium salicylate in a hydraulic fluid.

111-04-200~01 14:49 FROM SHELL INTERNATIONAL TO 800498923994465 EP 00000325E

Hydraulic fluids need to meet special requirements as they both lubricate and transmit pressure. In order to meet these requirements, hydraulic fluids contain additives. Generally, hydraulic fluide contain antiwear/extreme pressure additives. The conditions for a hydraulic fluid are especially difficult in vane pumps where the vane tips slide against the casing at high speed under heavy load and at high temperature. Further, the increasingly high operating temperatures of modern hydraulic equipment requires the fluid to be thermally stable to avoid the formation of deposits and$ludge and to resist the corrosion of both ferrous and non-ferrous metals. The requirement of good thermal stability in hydraulic fluids containing an anti-wear additive, means that these additives are often used in combination with detergents. However, the calcium salts of alkylbenzene sulphonates, alkyl-naphthalene sulphonates, petroleum suiphonates, alkylphenates, alkyl suiphurized phenates or alkylsalicylates which are usually applied, give.
increased wear. The increased wear is especially marked in vane pumps, more specifically under low load conditions. Low load coziditions occur in practice when the pump.is idle.

= WO-A-97/10318 describes lubricants with improved acid neutralisation properties and excellent sludge' and wear performance in the Seq. 5E test. said lubricants are formed using a detergent system comprising one or more alkali or alkaline earth metal salts of an oil soluble organic acid selected from the group consisting of sulfonic acids, phenols, sulfurised phenols and AMENDED SHEET

11-04-2001 2001 14:49 FROM SHELL INTERNATIONAL TO 000498923994465 EP 000003258 carboxylic acids (including salicylate acids) wherein at least one metal salt,is overbased and the ratio of inorganic to organic salt present in the detergent system expressed as TBN from inorganic salts (the overbasing) to the total moles of organic salt is at least 2S00.

US-A-5 558 802 describes multigrade crankcase lubricants that have.good low temperature pumpability and low volatility.

US-A-5 558 802 states that neutral and overbased calcium'salts of organic acids have been found to increase low-temperature high shear viscosity when measured in a cold cranking simulator (CCS).
In order to produce lubricants with low CCS

viscosities under high shear rates (i.e- to allow the engine to crank more easily at lower temperatures and thus improve engine startability at those ambient temperatures), it is said in US-A-5 558 802 that these salts should be avoided altogether or used at a level not exceeding 0.0007 moles of calcium salt per 100 grams of lubricant.
WO-A-96/37582 describes lubricating oil compositions for internal-combustion engines, automatic transmission gearboxes, dampers, power steering units and the like.

Lubricating oils of described in WO-A-96/37582 comprise sulfoxymolybdenum dithiocarbamate having a specific alkyl group, zinc dialkyl dithiophosphate having a specific alkyl group, specific alkylsalicylate(s), and, if desired, succinimide 11-04-2001 ~01 14:50 FR01'1 SHELL C INTERNATIONAL 2001-10-11 containing boron, each in a predetermined amount intra lubricating oil base stock.
US-A-446291s describes a lubricating oil composition comprising a major proportion of a lubricating oil and a minor proportion of each of a Group II metal dithiophosphate and a composition having the formula:
R1O.CO.CR2(NR3R4).CR6R7.CO.OR5, where R1, R2, R3, R4, R5, R6 and R7 are hydrogen or a hydrocarbon radical containing 1-30 carbon atoms, and wherein R3 or R4 are independently selected from hydrogen, hydrocaxbon containing from 1 to 30 carbon atoms, and acyl containing from 1 to 30 carbon.

summary of the Ipvention it has now aurprisingly been found that hydraulic fluid containing a combination of magnesium salicylate and zinc dithiophosphate, gives improved performance at low load compared with a combination containing calcium salicylate. Additionally, it was found that this combiiiation also provides greater thermal stability.

Less sludge and deposits are formed when subjecting the hydraulic fluid of the present invention to the Cincinnati Milacron Thermal stability Test.

The present invention provides a hydraulic flu3.d comprising a lubricant base oil in combination with (a) from 0.001 to 5twt of magnesium aalicylate, (b) from 0.01 to 8twt of zinc dithiophosphate, which amounts are based on total weight of hydraulic fluid.

AMENDED SHEET

- 5a -In one aspect of the invention, there is provided use of a composition comprising a lubricant base oil in combination with (a) from 0.001 to 5 %wt of magnesium salicylate, (b) from 0.01 to 8 %wt of zinc dithiophosphate to give improved wear performance at low load in a hydraulic fluid, the composition being free of calcium salicylate.
In another aspect of the invention, there is provided use of an additive package comprising magnesium salicylate, zinc dithiophosphate and optionally a dicarboxylic acid or its mono- or di-ester or its monoamide or di-amide or imide containing in total between 4 and 70 carbon atoms, wherein the weight ratio of said magnesium salicylate to said zinc dithiophosphate is from 1:5 to 1:100 and the weight ratio of magnesium salicylate to said dicarboxylic acid or its mono- or di-ester or its monoamide or di-amide or imide is from 1:0 to 1:50, the package being free of calcium salicylate, to prepare a hydraulic fluid with improved wear performance at low load.
In a further aspect of the invention, there is provided a hydraulic fluid comprising a composition comprising a lubricant base oil in combination with (a) from 0.001 to 5 %wt of magnesium salicylate, (b) from 0.01 to 8 %wt of zinc dithiophosphate, and (c) from 0.001 to 5 %wt of a compound according to the following formula I:

R3R4C-COOR, 1 (I) in which R, and R2 are each hydrogen or alkyl or hydroxyalkyl of 1 to 30 carbon atoms;
R3, R4 and R5 are each hydrogen or alkyl or hydroxyalkyl of 1 to 4 carbon atoms: X is CH or N and R6 and R7 are each hydrogen, alkyl or alkenyl of I to 30 carbon atoms, or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms.

- 5b -In a still further aspect of the invention, there is provided an additive package for preparing a hydraulic fluid, which additive package comprises: magnesium salicylate, zinc dithiophosphate and a compound according to the following fonnula I:

R3R4C-COOR, I (I) in which R, and R2 are each hydrogen or alkyl or hydroxyalkyl of I to 30 carbon atoms;
R3, R4 and R5 are each hydrogen or alkyl or hydroxyalkyl of 1 to 4 carbon atoms; X is CH or N and R6 and R7 are each hydrogen, alkyl or alkenyl of 1 to 30 carbon atoms, or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms, wherein the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100 and the weight ratio of magnesium salicylate to said compound of formula Iisfroml:>Oto1:50.

11-OA-2001301 14:50 FROM SHELL INTERNATIONAL TO 060498923994465 EP 000003258 tZnt3ilesi DPa -=; ation of the Inyg i nn The magnesium salicylate for use in the present invention can be either neutral or overbased. The expression "overbased" is equivalent to "basic", "superbased", "hyperbased" and "high-metal containing salts". These magnesium salicylates contain an excess metal content compared to the amount of metal which would be present accbrding to the stoichiometry of the metal and the salicylie acid reacted with the metal.

Processes for making such neutral and basic metal salts are well known in the art. Neutral salte can be made by heating a mineral oil solution of an acidic organic compound with a stoichiometric equivalent amount of a metal neutralizing agent such as the metal oxide, hydroxide, carbonate, bicarbonate, or sulfide at a temperature above 50 C and filtering the resulting mass. Basic salts are made similarly with the exception that a stoichiometric excess of the metal is used.

Preferably, overbased magnesium salicylate is used.
20= These compositions can be characterized by their total base number (TBN). The total base number is preferably at least 100 mg KOH/g, more preferably at least 200 mg KOH/g, most preferably at least 300 mg KOH/g. The total = base number is preferably at most 600 mg KOH/g. Another method of characterising overbased magnesium salicylates is by the magnesium content relative to the stoichiometrically equivalent amount of magnesium based on the amount of total acid present. Overbased magnesium salicyclates for use in the present invention preferably have a magnesium content of more than 500% of the stoichiometrically equivalent amount of magnesium based on the amount of total acid present, more preferably at least 550%.
The salicylate can be either substituted or unsubstituted. Suitable substituents include aliphatic groups containing from 1 to 40 carbon atoms and optionally containing one or more oxygen and/or nitrogen atoms, and hydroxy groups. Preferred substituents are alkyl= groups containing from 6 to 30 carbon atoms, preferably from 12 to 20 carbon atoms. Preferably, the substituents are linear. The salicylate can contain from 1 to 4 substituents, preferably from 1 to 3, most preferably 1 or 2 subatituents. Most preferably, the salicylate is substituted by 1 linear alkyl group containing from 14 to 18 carbon atoms.

Generally, mono-alkyl salycylic acids are prepared by alkylation of phenol and subsequent carboxylation.
Therefore, a small amount (generally at most 20 Wmol) of dialkyl salicylate and unsubstituted salicylate can be present in the mono-alkyl salicylate.

Magnesium salicylate which can be used in the present invention, is commercially available. A suitable commercial product is SAP 007 (ex Infineum). The magnesium salicylate will usually be supplied in combination with mineral oil. The amounts referred to in this document, relate to the compound per se without mineral oil.

A process by which suitable magnesium salicylate can be prepared, has been described in US-A-4,627,928.

~
11-04-2(101 2001 14:50 FROM SHELL INTERNATIONAL TO 000498923994465 EP

- e -Zinc dithiophosphate ie well known in the art.
Preferred dithiophosphate for use in the present invention.are represented by the following formula O-R' R8 - O- P - S (formula II) S"

in which R and R8 can be either hydrogen or an aliphatic group. The aliphatic group is preferably a hydrocarbyl group. The hydrocarbyl group optionally contains an acid, a hydroxyand/or ari ester group. The hydrocarbyl group is preferably an alkyl containing up to 12 carbon atoms optionally further containing an acid, a hydroxy and/or an ester group. The hydrocarbyl substituted dithiophosphate can contain 1 or 2 aliphatic groups, or it can be a mixture of dithiophosphates having 1 aliphatic group and dithiophosphates having 2 aliphatic groups. Preferably, the zinc dithiophosphate is a zinc dialkyl dithiophosphate.

Substituted dithiophosphates can contain primary, secondary and/or tertiary aliphatic substituents, i.e.
substituents which are attached to the oxygen atom via a carbon atom attached to one, two or three further carbon = 25 atoms, respectively. In the present invention, it is preferred that at least 60 twt of the aliphatic, preferably alkyl, substituents of the zinc dithiophosphate, are primary aliphatic substituents.
More preferably at least 80 %wt of the aliphatic substituents are primary aliphatic substituents.

1 1-04-2001 2001 14 : 51 FROt'1 SHELL I NTERNAT I ONAL TO 000498923994465 EP

suitable zinc dithiophosphates which are commercially available include Elco 108 (ex Elco Corporation), Lz 1375 (ex Lubrizol), OLOA 4269Q (ex Oronite) and HITEC 680 (ex Ethyl).

The amount of magnesium salicylate which is present is preferably at most 1twt, based on total composition, more preferably at most 0.5 twt, most preferably at most 0.3 %wt. The amount of magnesium salicylate which is present is preferably at least 0.001 %wt, more preferably -at least 0.005 %wt more preferably at least 0.01 %wt, most preferably at least 0.015 twt. The amount of zinc dithiophosphate which-is present is preferably at most 5 %wt, based on total composition, more preferably at most 3 %wt, moet preferably at most 1 %wt: The amount of zinc dithiophosphate which is present is preferably at least 0.05 %wt, more preferably at least 0.7 &wt, most preferably at least 0.1 %wt.
The hydraulic fluid can further comprise (c) from 0.001 to 5 %wt of a rust inhibitor based on total.
composition, preferably a carboxylic acid or ester or amide or imide containing compound, more preferably a dicarboxylic acid or its mono- or di-eater or its mono-amide or di-amide or imide, more preferably a = dicarboxylic acid or its ester, amide or imide containing a total number of between 4 and 70 carbon atoms. More preferably, the number of carbon atoms of the compound is between 20 and 40. A preferred compound is an aliphatic succinimide or succinic diamide. Such compounds have been described in EP-A-776964.

Most preferably, (c) is from 0.001 to 5 %wt of a compound according to the following formula I

I (1) in which Rl and R2 are each hydrogen or alkyl or hydroxyalkyl of 1 to 30 carbon atoms;
R3, R4 and R5 are each hydrogen or alkyl or hydroxyalkyl of 1 to 4 carbon atoms; X is CH or N and R6 and R7 are each hydrogen, alkyl or alkenyl of 1 to 30 carbon atoms, or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms. Preferably, RI and R2 are each an alkyl of from 3 to 6 carbon atoms, R3, R4 and R5 are each hydrogen, X is N and R6 and R7 are each an alkyl of 15 to 20 carbon atoms or an acyl group derived from a saturated or unsaturated dicarboxylic acid containing 4 to 10 carbon atoms, at least one of R6 and R7 being an acyl group. Especially preferred is aspartic acid, N-(3-carboxy-l-oxo-2-propenyl)-octadecyl-bis(2-methylpropyl) ester.
Such aspartic acid esters are commercially available; processes for preparing such compounds having been described in EP-A-434 464.
The weight ratio in which the compounds of the present invention are present can differ within wide ranges. Preferably, the hydraulic fluid comprises magnesium salicylate, zinc dithiophosphate and optionally a rust inhibitor in such quantities that the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100 and the weight DOCSMTL: 2002456\I

~ 11-04-2001 001 14:51 FROM SHELL INTERNATIONAL TO 000498923994465 EP

ratio of magnesium salicylate to rust inhibitor is from 1:0 to 1:50.

The lubricant base oils present in the hydraulic fluid of the present invention, can be any base fluid which is suitable for use in hydraulic fluids. The base oil can be a natural or a synthetic lubricant base oil, or a mixture thereof. The natural oil can be an animal oil or vegetable oil, such as.lard oil or castor oil, or a mineral oil such as liquid petroleum oils and solvent treated or acid treated mineral lubricating oil of the paraffinic, naphthenic, or mixed paraffinic/naphthenic type which may be further refined by hydrocracking and hydrofinishing processes and/or dewaxing. Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerised and interpolymerised olefins. A suitable base oil contains poly-alpha-olefins, such as polydecene. Preferably, the base oil is a hydrocarbon base oil. More preferably, the base fluid is a mineral oil which contains less than 10 t by weight of aromatic compounds, preferably less than 5t by weight, most preferably less than 3.0 $ by weight, measured according to DIN 51378. It is further preferred that the base oil contains less than 1.0 twt of sulphur, calculated as elemental sulphur, preferably less than 0.1 kwt, more preferably less than 0.05 twt, measured according to ASTM D 4045. Such mineral oils can be prepared by severe hydroprocessing. Preferably, the lubricating oil has a kinematic viscosity in the range of from 5 to 220 cSt at 40 C, more preferably of from 10 to 200 cSt, most preferably of from 20 to 100 cst.

11-04-2001,001 14:51 FROM SHELL INTERNATIONAL 2001-10-11 The hydraulic fluid according to the present invention can contain further additives usually present in hydraulic fluids, such as pour point depressants, anti-foam agents and demulsifier. Pour point depressants generally are high molecular weight polymers such as alkylaromatic polymers and polymethacrylates. As anti-foam agents, silicone polymers and/or polymethacrylates are generally ueed. Demulsifiers which are generally applied are polyalkylene glycol ethers. Furthermore, further detergents such as sulphonates and phenates, metal deactivators, antioxidants such as phenolic compounds, diphenyl amines and phenyl naphthyl amines, ashless anti-wear agents and/or ashless dispersants, such as succinimides, can be present.

The hydraulic fluids of the present invention contain magnesium salicylate. For specific applications, it can be advantageous that the fluids contain a combination of magnesium and calcium salicylate.
The components of the present invention can be added per se to a lubricating oil, or they can be mixed into an additive package before being added to the lubricating oil. The.additive package for preparing the hydraulic fluid preferably contains magnesium salicylate, zinc dithiophosphate and optionally a dicarboxylic acid or its mono- or di-ester or its mono-amide or di-amide or imide containing in total between 4 and 70 carbon atoms,.wherein the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100 and the weight ratio of magnesium salicylate to AMENDED SHEET

dicarboxylic acid or its mono- or di-ester or its mono-amide or di-amide or'imide is from 1:0 to 1:50.
E;XAI~PLES
Formulations were prepared containing the following additives:

Magnesium salicylate: highly overbased magnesium alkylealicylate having a total base number of 337 mg KOH/g (SAP 007 ex INFINEUM) and a magnesium content of about 7504 of the stoichiometrically equivalent amount of magnesium based on the amount of total aci4, containing 40 %wt of mineral oil Calcium salicylate: overbased calcium alkylsalicylate having a total base number of 168 mg KOH/g (SAP 001 ex INFINEUM), containing 40 -1wt of mineral oil.

The amounts of metal salicylate mentioned in Table 1, are the amounts of metal salicylate per se, without mineral oil.

Zinc dithiophosphate: zinc di(ethyl-hexyl)dithiophosphate (Elco 108 ex Elco corporation) Compound according to formula I: aspartic acid, N-(3-carboxy-l-oxo-2-propenyl)-N-octadecyl , bis(2-rnethylpropyl)ester.

These compounds were added to a lubricant base oil = containing less than 1.0 & by weight of aromatic compounds, legs than 0.05 %wt of sulphur, calculated as elemental sulphur and having a kinematic viscosity in the range of 32 cSt (at 40 ) (ISO viscosity grade 32).

-11-04-2001 001 14:52 FROM SHELL INTERNATIONAL-lo-11 TO 000498923994465 EP

The compositions obtained are described in Table 1.
The amounts are of the compounds per se, excluding the mineral oil.

Compositions 1 and 2 were used in Vickers V104C vane pump tests. A new pump cartridge was used for each test.
The test duration was 250 hours, performed at a fluid temperature of about 66 C, at a fluid outlet pressure of 35 bar (3.5 MPa) and at a pump speed of 1450 revolutions per minute. The results are described in Table 1.

T b~1 ~1 Composition 1 Composition 2 magnesium salicylate 0.06 twt -calcium salicylate - 0.06 twt zinc dithiophosphate 0.37 twt 0.37 twt compound of formula I 0.10 %wt 0.10 $wt lubricant base oil balance balance Low load test ring weight loss (mg) 13 96 vane weight loss (mg) 2 1 total weight loss (mg) 15 97 From Table 1 it is clear that better protection against wear is provided by a composition containing magnesium salicylate (1), than by a composition containing calcium salicylate (2). The ring weight loss and total.weight loss incurred when using composition 1 is significantly less than that incurred when composition 2 is employed-The vane weight loss is similar for both compositions.

AMENDED SHEET

Claims

CLAIMS:

1. Use of a composition comprising a lubricant base oil in combination with (a) from 0.001 to 5 %wt of magnesium salicylate, (b) from 0.01 to 8 %wt of zinc dithiophosphate, wherein said composition is free of calcium salicylate, to give improved wear performance at low load in a hydraulic fluid.

2. Use of a composition according to claim 1, further comprising (c) from 0.001 to %wt of a dicarboxylic acid or its mono- or di-ester or its mono-amide or diamide or imide containing in total between 4 and 70 carbon atoms.

3. Use of a composition according to claim 2, in which (c) is a compound according to the following formula I:

in which R1 and R2 are each hydrogen or alkyl or hydroxyalkyl of 1 to 30 carbon atoms;
R3, R4 and R5 are each hydrogen or alkyl or hydroxyalkyl of 1 to 4 carbon atoms; X is CH or N and R6 and R7 are each hydrogen, alkyl or alkenyl of 1 to 30 carbon atoms, or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms.

4. Use of a composition according to claim 1, comprising said magnesium salicylate and zinc dithiophosphate in such quantities that the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100.

5. Use of a composition according to claim 2, comprising said magnesium salicylate, zinc dithiophosphate and the compound of (c) in such quantities that the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100 and the weight ratio of magnesium salicylate to said compound of (c) is from 1: >
0 to 1:50.

6. Use of a composition according to claim 3, comprising said magnesium salicylate, zinc dithiophosphate and said compound of formula I in such quantities that the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1.100 and the weight ratio of magnesium salicylate to said compound of formula I is from 1: >
0 to 1:50.

7. Use of a composition according to any one of claims 1 to 6, which composition further comprises at least one of a pour point depressant, an anti-foam agent and a demulsifier.

8. Use of an additive package comprising magnesium salicylate, zinc dithiophosphate and optionally a dicarboxylic acid or its mono- or di-ester or its monoamide or di-amide or imide containing in total between 4 and 70 carbon atoms, wherein the weight ratio of said magnesium salicylate to said zinc dithiophosphate is from 1:5 to 1.100 and the weight ratio of magnesium salicylate to said dicarboxylic acid or its mono- or di-ester or its monoamide or di-amide or imide is from 1:0 to 1:50, and wherein said package is free of calcium salicylate, to prepare a hydraulic fluid with improved wear performance at low load.

9. A hydraulic fluid comprising a composition comprising a lubricant base oil in combination with (a) from 0.001 to 5 %wt of magnesium salicylate, (b) from 0.01 to 8 %wt of zinc dithiophosphate, and (c) from 0.001 to 5 %wt of a compound according to the following formula I:

in which R1 and R2 are each hydrogen or alkyl or hydroxyalkyl of 1 to 30 carbon atoms;
R3, R4 and R5 are each hydrogen or alkyl or hydroxyalkyl of 1 to 4 carbon atoms: X is CH or N and R6 and R7 are each hydrogen, alkyl or alkenyl of 1 to 30 carbon atoms, or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms.

10. A hydraulic fluid according to claim 9, comprising said magnesium salicylate, said zinc dithiophosphate and said compound of formula I in such quantities that the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100 and the weight ratio of magnesium salicylate to said compound of formula I is from 1:> 0 to 1:50.

11. A hydraulic fluid according to claim 9 or 10, which fluid further comprises at least one of a pour point depressant, an anti-foam agent and a demulsifier.

12. Additive package for preparing a hydraulic fluid which additive package comprises: magnesium salicylate, zinc dithiophosphate and a compound according to the following formula I:

in which R1 and R2 are each hydrogen or alkyl or hydroxyalkyl of 1 to 30 carbon atoms;
R3, R4 and R5 are each hydrogen or alkyl or hydroxyalkyl of 1 to 4 carbon atoms; X is CH or N and R6 and R7 are each hydrogen, alkyl or alkenyl of 1 to 30 carbon atoms, or an acyl group derived from a saturated or unsaturated carboxylic acid of up to 30 carbon atoms, wherein the weight ratio of magnesium salicylate to zinc dithiophosphate is from 1:5 to 1:100 and the weight ratio of magnesium salicylate to said compound of formula I is from 1:> 0 to 1:50.