Zinc di--alkyl dithio--phosphate (ZnDTP) compounds are widely used in engine and transmission oils both as anti--oxidants and as anti--wear additives. However@ recent work has shown that many anti--wear additives appear to have a detrimental effect on the resistance of gears and other contacting components to various types of rolling contact fatigue@ including micropitting. In the present paper we examined the effect of the presence of a secondary C6 ZnDTP in a low viscosity synthetic base oil on the resistance to micropitting and wear of carburised steel rollers@ using a triple-contact disk tester. It was found that the additive caused severe micropitting and associated wear@ whereas the pure base oil did not give rise to any micropitting. It was further found that the additive was not detrimental unless it was present during the first 100 000 cycles of the test when it was found to exert a strong effect on the development of roughness on the counter-rollers. It is concluded that the additive is detrimental to micropitting resistance because it retards wear-in of the contact surfaces@ favouring the development of damaging fatigue cracks. This contrasts with some earlier speculation that suggested a direct chemical effect could be responsible.