Trunk piston marine engine crackcase lubrication is effected by a composition made by blending minor amounts of (A) a metal dithiophosphoric acid salt additive component comprising 50 mole % or more of a zinc di (C6 primary alkyl) dithiophosphate, and (B) an overbased metal detergent additive component, with (C) a major amount of an oil of lubricating viscosity.