A lubricating oil composition suitable for a trunk piston marine diesel engine. The lubricating oil composition comprises an admixture of: (A) an oil of lubricating viscosity, in a major amount; and (B) an oil-soluble complex overbased metal detergent additive, in a minor amount. The composition is substantially dispersant-free or contains 1, or less than 1, preferably less than 0.5, mass % based on the mass of the compositions, of a dispersant. The composition has a TBN in the range of 3.5 to 100, preferably 8 to 100.