The basic physicochemical indexes of M-10-D-2(m) engine oil change much less intensively in a gas engine than in a diesel. Low accumulation of elements-indicators of parts wear-and fouling of parts by carbon and varnish deposits were noted. Formation of ash deposits in the valves was more important. Dithiophosphate additives were depleted similarly in both engines. Information on the requirements for special oils for gas engines is reported.