Lubricating oil compositions are used for the smooth operation of internal combustion engines, power transmission components including automatic transmissions, shock absorbers and power steering devices and gears. The engine oils for internal combustion engines in particular serve to (i) lubricate various sliding interfaces eg between the piston ring and cylinder liner, in bearings of the crank shaft and the connecting rod, and in the valve driving mechanism including cams and valve lifters, (ii) cool the engine, (iii) clean and disperse the combustion products and (iv) prevent corrosion and consequent rust formation. The stringent requirements for high performance engines in recent years have meant greater demand from lubricants used in such engines. Lubricating oils used in such engines usually deteriorate due to oxidation by oxygen and/or nitrogen oxides (NOx) contained in the blow-by gas (ie gas leaking from the combustion chamaber gases into the crankcase at the piston/cylinder interface). The concentration of NOx increases in the blow- by gas with increasing demand in performance of the engine. The deleterious effects of oxidation can be and have been mitigated by the use of various additives including antioxidants and by the use of base stocks which are relatively more stable to oxidation.

The control of oxidation processes has been improved to some extent by the use of lubricating compositions which comprise a Group II, III or IV basestocks which are relatively high in saturated hydrocarbons (hereafter"saturates") in preference to Group I basestocks which are relatively low in saturates because of the relatively high propensity of Group I base oils to oxidation. Group II, III and IV basestocks are typically more expensive than the conventional Group I basestocks. To extend the use of Group I basestocks for future engine oil products, it is important to identify suitable additives to enhance their properties. A feature of the present invention is the improvement of the oxidative stability of Group I basestocks.

It has now been found that it is possible to achieve the desired degree of oxidation stability from Group I base oils by judicious choice of antioxidants without recourse to relatively expensive Group II, III and/or IV basestocks.

Accordingly, the present invention is a lubricating oil composition comprising a major amount of a Group I base stock and an antioxidant comprising an oil soluble trinuclear organomolybdenum compound of the generic formula :

Mo3S,- (Q) (I) wherein x is from 4 to 10, preferably 7, and Q is a core group, which may be a ligand.

The lubricating oil compositions of the present invention are those that comprise a major amount of a Group I basestock as defined in API 1509. Thus, for instance the Group I basestock may be mineral oils or blends thereof having a KVloo of 2-50 cSt, a viscosity index of 80-120, a saturates content of <90% w/w, an aromatics content of > 10% w/w and a sulphur content of > 0. 03% w/w (300 ppm). Group I basestocks are produced by solvent extration followed by dewaxing. Specific examples of Group I basestock include inter alia solvent neutral basestocks such as MCT 30 (94 VI, 78% w/w saturates, 0. 28% w/w sulphur) ; while hydrocracked distillates and raffinates which have been dewaxed, eg by conventional or catalytic dewaxing techniques, represent Group II and Group III base stocks with a sulphur content of less than 0. 03% w/w and an aromatics content of < 10% w/w such as RLOP 500R (106 VI, >97% w/w saturates, and Mobil Jurong 500N (99VI, >97% w/w saturates). Polyalphaolefins represent Group IV basestocks.

According to a further embodiment, the present invention is a method of improving the oxidative stability of a lubricant composition, said composition comprising a Group I base stock which has a kinematic viscosity at 100°C (KVloo) from about 2 cSt to 50 cSt (2 x 10-6 to 50 x 10-6 m2/sec), a saturates content of at less than 90% w/w, an aromatics content of > 10% w/w and a sulphur content of > 0. 03% w/w (300 ppm) said method comprising adding to the basestock an effective amount of an antioxidant comprising an oil soluble trinuclear organomolybdenum compound of the generic formula : Mo3Sx-(Q) (D wherein x is from 4 to 10, preferably 7, and Q is a core group, which may be a ligand.

The trinuclear molybdenum compounds are of formula (I) Mo3Sx-(Q) (0 wherein x is from 4 to 10, preferably 7, and Q is a core group. Trinuclear molybdenum compounds are relatively new and are claimed and described in our prior published US-A- 5, 906, 968. The matter disclosed in this prior US patent on the structure, preparation and properties of the trinuclear molybdenum compounds is incorporated herein by reference. In these compounds the core group (Q) may be a ligand capable of rendering the organomolybdenum compound of formula (I) oil soluble and ensuring that said molybdenum compound is substantially charge neutral. The core group (Q) is generally associated with suitable ligands such as Ly wherein L is the ligand and y is of a sufficient number, type and charge to render the compound of formula (I) oil soluble and to neutralise the charge on the compound of formula (I) as a whole. Thus, more specifically, the trinuclear molybdenum

compound used in the compositions of the present invention may be represented by the formula (II) : Mo3sxLy Ly (II) The ligands"L"are suitably dihydrocarbyl dithiocarbamates of the structure (-S2CNR2) wherein the dihydrocarbyl groups, R2 impart oil solubility to the molybdenum compound. In this instance, the term"hydrocarbyl"denotes a substituent having carbon atoms directly attached to the remainder of the ligand and is predominantly hydrocarbyl in character within the context of this invention. Such substituents include the following : (1) hydrocarbon substituents, ie, aliphatic (for example alkyl or alkenyl), alicyclic (for example cycloalkyl or cycloalkenyl), aromatic-, aliphatic-and alicyclic- substituted aromatic nuclei and the like, as well as cyclic substituents wherein the ring is completed through another portion of the ligand (that is, any two indicated substituents may together form an alicyclic group) ; (2) substituted hydrocarbon substituents, ie, those containing nonhydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbyl character of the substituent. Those skilled in the art will be aware of suitable groups (eg halo (especially chloro), amino, alkoxyl, mercapto, alkylmercapto, nitro, nitroso, sulphoxy etc.) ; and (3) hetero substituents, ie, substituents which, while predominantly hydrocarbon in character within the context of this invention, contain atoms other than carbon present in a chain or ring otherwise composed of carbon atoms.

The hydrocarbyl groups are preferably alkyl (e. g, in which the carbon atom attached to the remainder of the ligand"L"is primary, secondary or tertiary), aryl, substituted aryl and/or ether groups.

Importantly, the hydrocarbyl groups of the ligands should be such that they have a sufficient number of carbon atoms to render the compound (I) soluble or dispersible in the oil to which the trinuclear organomolybdenum compound containing the ligand is added.

The total number of carbon atoms present among all of the hydrocarbyl groups of the organomolybdenum compounds'ligands is suitably at least 21, preferably at least 25, more preferably at least 30 and even more preferably at least 35, typically e. g., 21 to 800. For instance, the number of carbon atoms in each hydrocarbyl group will generally range from 1 to 100, preferably from 1 to 40 and more preferably from 3 to 20.

The antioxidant which comprises the trinuclear organomolybdenum compounds will form a minor component of the total lubricant composition. For example, the trinuclear organomolybdenum compounds will comprise from about 0. 05 to 5. 00% w/w of the total composition, ie the molybdenum metal is suitably present in an amount of about 25 to 3000 ppm, preferably from about 50 to 1000 ppm, and more preferably from about 100-500 ppm of the total composition.

The antioxidant trinuclear molybdenum compound of the present invention can be used with any of the conventional dispersants used hitherto in the lubricating compositions.

Examples of such dispersants include inter alia the polyalkylene succinimides, Mannich condensation products of polylalkylphenol-formaldehyde polyamine and boronated derivatives thereof. However, it is preferable to use ashless dispersants such as the ashless succinimides, especially the polyisobutenyl succinimides of a polyamine such as eg tetraethylenepentamine or its homologues, benzylamine ashless dispersants, and ester ashless dispersants. The dispersants are generally used in the compositions of the present invention in an amount ranging from about 2-10% by weight based on the total weight of the lubricant composition, preferably from about 4-8% by weight.

In general, these lubricating compositions may include additives commonly used in lubricating oils especially crankcase lubricants, such as antiwear agents, detergents, dispersants, rust inhibitors, viscosity index improvers, extreme-pressure agents, friction modifiers, corrosion inhibitors, emulsifying aids, pour point depressants, anti-foams and the like.

A feature of the present invention is that lubricant compositions comprising relatively low saturates base oils and trinuclear organomolybdenum compounds provide unexpected improvement in oxidation control and significant benefits in fuel economy.

The present invention is further illustrated with reference to the following Examples : EXAMPLES General Procedure : A series of Test oils were prepared. These oils were then tested in a bench oxidation test which was conducted at 165°C under a mixed nitrogen/air flow, with 40 ppm iron from added ferric acetylacetonate as catalyst. The flow rates of air and nitrogen were controlled at 500 ml/min and 350ml/min respectively.

In these tests the following commercial materials have been used : Paranox@ 106 is a polyisobutenylsuccinimide dispersant (ex Infenium, Linden, NJ) Molyvan 822 is a dinuclear molybdenum dithiocarbamate (ex R T Vanderbilt Co) PDN 5203 is an experimental trinuclear molybdenum dithiocarbamate containing 5% w/w molybdenum.

Examples A-C Comparison between Group I and Group II base oils : The compositions of the test oils used in these Examples and their respective changes in viscosities after a 48 hour oxidation test are given in Table 1 below : TABLE 1 Example A B C MCT 30 (% wt) 93.0 - - RLOP 500R (% wt) 93. 0 Mobil Jurong 500N (% wt) 93. 0 Paranox (D106 (% wt) 6. 0 6. 0 6. 0 Mo3-dithiocarbamate* 1. 0 1. 0 1. 0 Fresh Oil KVloo, cSt 12. 99 12. 87 12. 60 Used Oil KVloo, cSt 14. 3 25. 03 46. 25 % Increase 10. 08 94. 48 267. 06 * containing 11. 5% wt of molybdenum From the above it can be seen that the trinuclear molybdenum dithiocarbamate unexpectedly gives a better performance in Group I low saturates base oils than in the Group II high saturates base oils.

EXAMPLES D-1 Comparison between a trinuclear molybdenum compound and a dinuclear molybdenum compound The compositions of the test oils in Examples D-1 and their respective changes in viscosity data after a 32-hour oxidation test on each are shown in Table 2 below :

TABLE 2 Example D E F G H MCT 30 (% wt) 94. 0 93. 0 93. 0 RLOP 500R (% wt) - - - 94.0 93. 0 93. 0 Paranox (D106 (% wt) 6. 0 6. 0 6. 0 6. 0 6. 0 6. 0 PDN 5203 (% wt)-1. 0--1. 0- Molyvan# 822 (% wt) - - 1.0 - - 1. 0 Fresh Oil KVloo, cSt 12.98 12.97 12.96 12.68 12. 69 12. 68 Used Oil KV100, cSt 45.34 13.96 17.38 35.01 21. 98 24. 92 % Increase 249 7. 63 34. 1 176 73. 21 96. 53 Examples D-1 show that the trinuclear molybdenum dithiocarbamate gives significantly better performance than conventional dinuclear molybdenum dithiocarbamate in oxidation control. The performance of the trinuclear molybdenum compound is further enhanced in Group I base oils when compared with Group II base oils.

Examples J-M The changes in viscosity after a 48 hour oxidation test in Examples J-M is shown in Table 3 below : TABLE 3 Example J K L M MCT 30 (% wt) 93. 0 93. 0 RLOP 500R (% wt) - - 93. 0 93. 0 Paranox0106 (% wt) 6. 0 6. 0 6. 0 6. 0 PD5203 (% wt) 1. 0-1. 0- Molyvan#822 - 1.0 - 1. 0 Fresh Oil KV) oo, cSt12. 9813. 0012. 65 12. 71 Used Oil KV100, cSt 16. 19 24. 94 78. 99 79. 50 % Increase 24. 7 91. 8 524 526 Example J-M demonstrate that trinuclear molybdenum dithiocarbamates give a better performance over an extended period of time than the conventional dinuclear molybdenum dithiocarbamates in oxidation control of Group I base oils.