A trunk piston marine engine lubricant comprises in respective minor amounts (A) an overbased metal hydrocarbyl-substituted hydroxybenzoate detergent system, and (B) a hydrocarbyl-subsituted succinic acid anhydride, preferably made by halogen- or radical-assisted fractionalization processes, where the ratio of succinic anhydride to hydrocarbyl chains is in the range of 1.4 to 4. The lubricant, when used to lubricate such an engine fuelled by heavy fuel oil, exhibits improved control of asphaltene precipitation and deposition on engine surfaces.