Extreme pressure lubricant additive

A lubricating oil composition comprises a mineral lubricating oil, 1 to 20% wt. of a zinc C3-C20 alkyl dithiophosphate and 1 to 20% wt. of a paraffinic hydrocarbon having both chlorine and sulphur or sulphur-containing substituents such that the sulphur content is 3 to 10% wt. and the chlorine content is 10 to 40% wt. Preferred mineral oils for use as gear oils have a V.I. of 40 to 130 and a viscosity of 60 to 300 SUS/210 DEG F. Examples of dithiophosphates are zinc diisopropyl-, dihexyl-, dioctyl-, dicapryl-, and dioctadecyl-dithiophosphate. The chlorine- and sulphur-substituted hydrocarbon may be a C12-C30 aliphatic hydrocarbon e.g. kerosine, gas oil, lubricating oil, paraffin wax or petroleum. The chlorine and sulphur may be introduced by treatment with a sulphur chloride e.g. the mono- or di-chloride, or by chlorination followed by treatment with sulphur; an inorganic sulphide such as sodium sulphide or polysulphide or sulphur plus sodium hydroxide; or an organic sulphur compound e.g. a thiocarbonate, xanthate or thiocyanate. Examples comprise a mineral lubricating oil having a viscosity of 1094 SUS/100 DEG F. and a V.I. of 90; zinc dihexyl of C4-C5 alkyl dithiophosphate; a material obtained by mixing 0.7 volumes of 40% chlorinated kerosine and 1 volume of 33% chlorinated wax and treating the mixture with sodium hydrosulphide and sulphur to obtain a product containing 30% wt. chlorine and 6% wt. sulphur; and, optionally, the barium salt of a hydrolysed P2S5 treated olefin mixture. The compositions claimed p in Specification 805,553 are disclaimed.