Abstract: Lubricant additives comprising as overbased metal hydrocarbyl-substituted hydroxybenzoate detergent dispersed in diluent and a polyalkenyl-substituted carboxlic acid anhydride dispersed in a diluent oil containing 50 mass % or more of a basestock containing greater than or equal to 90% saturates and less than or equal to 0.03% sulphur or a mixture thereof are blended, in minor amounts, with a high saturates content oil of lubricating viscosity, in a major amount, to give trunk piston marine engine lubricating oil composition for a medium-speed four-stroke compression-ignited marine engine.

Abstract: Trunk piston marine engine lubrication comprising a major amount of (A) an oil of lubricating viscosity that comprises an oil-soluble ester basestock, or that comprises greater than 0.1 to less than 90 mass % of an oil-soluble ester basestock and, as 50 mass % or more of the remainder of the oil of lubricating viscosity, a high saturates basestock; (B) a minor amount of a metal alkyl salicylate detergent; and (C) greater than 0.1 to less than 10 mass %, of an oil-soluble polyalkenyl-substituted carboxylic acid anhydride, wherein the or at least one polyalkenyl group is derived from a polyalkene having a number average molecular weight of from 200 to 3,000. Asphaltene precipitation in the lubricant composition, caused by the presence of contaminant heavy fuel oil, is prevented or inhibited.