A lubricant composition of an oil of lubricating viscosity, an ashless condensation reaction product of an olefin polymer, having a number average molecular weight of 2,000 to 70,000, comprising carboxylic acid or equivalent functionality grafted onto the polymer backbone, with a monoamine or a polyamine often having a single primary amino group; a succinimide dispersant; and an overbased metal detergent, in an amount such that the total base number of the lubricant composition is less than 6.5, exhibits good sludge prevention performance in a gasoline engine.