Method for reducing piston deposits in a marine diesel engine

A method of reducing the incidence of deposits on the pistons of a 4-stroke marine diesel engine during operation of the engine when it is fuelled with a marine residual fuel meeting the ISO 8217 2017 fuel standard for marine residual fuels and having a sulphur content of more than 0.1% and less than 0.5% by mass, based on the mass of the fuel, the method comprising lubricating the engine using a lubricating oil composition comprising:

• (a) at least 50% by mass, based on the mass of the composition, of an oil of lubricating viscosity;
• (b) 5 to 25% by mass, based on the mass of the composition, of an oil-soluble or oil-dispersible alkali metal or alkaline earth metal salicylate detergent, or a mixture of two or more oil-soluble or oil-dispersible alkali metal or alkaline earth metal salicylate detergents, the or each oil-soluble or oil-dispersible alkali metal or alkaline earth metal salicylate detergent having a total base number of (TBN) as measured by ASTM D2896 of from 50 to 500 mg KOH/g;
• (c) 0.1 to 10% by mass, based on the mass of the composition of one or more oil-soluble or oil-dispersible ashless dispersants;
• and optionally,
• (d) 0.1 to 10% by mass, based on the mass of the composition of a polyalkylene-substituted succinic anhydride,
• wherein the lubricating oil composition provides the engine with reduced asphaltene agglomeration and/or reduced deposits on piston and ring assemblies, as compared with a lubricating oil composition lacking component (c) and/or component (d).