1 to about 20 0 by weight of a dialkyl hydrogen phosphite or a monoalkyl hydrogen phosphite to increase the phosphorous content of the resulting reaction product to improve anti friction characteristics wherein the alkyl moiety of dialkyl hydrogen phosphite or monoalkyl hydrogen phosphite is independently selected from a saturated straight or branched chain alkyl group having from two to 20 carbon atoms this example illustrates the synthesis of an inventive ep additive using a mixture of rapeseed oil hear and linseed oil both unsaturated triglyceride oils as the base oil and phosphorous pentasulfide as the ps reactant approximately 500 g of a mixture of hear and linseed oil in a ratio of 60 hear oil to 40 linseed oil by weight into a reactor that was heated to 150 u00b0 c phosphorous pentasulfide 17 9 g was slowly fed into the reactor over four hours of time this was the equivalent of a 1 by weight on a phosphorous basis of p 2 s 5 it was important not to raise the temperature much above 150 u00b0 c as this will cause the formation of poly sulfide chains causing the product to increase viscosity and degrade thermal stability of the product phosphorous pentasulfide was fed continuously as the solids were observed to dissolve after the reactants are added the temperature of the reaction mixture was raised to 200 u00b0 c over a minimum time of 30 minutes and the reaction continued for 2 hours the product was cooled under nitrogen gas at about 100 u00b0 c after which air exposure was allowed and the product was evaluated and analyzed the products were subject to ir scans to determine the presence of new bond formations fig1 scan d is a control ir scan of.