top cylinder cleaner like seafoam is good. DO NOT USE ENGINE FLUSH, it does nasty shit like pick up little pieces of casting flash and plugs up your oil pickup tube. the seafoam gets carbon off of the top of the pistons and helps prvent detonation. yearly maintenece should be filters tune up fluid change etc. always use premium quality fluids and filters. I don't use synthetics but I do use good additives from lucas oil products works out to the same price either way