Four stroke engines are lubricated by oil held in oil sump. The oil is distributed through the engine by splash lubrication or pressurized lubrication. These systems maybe alone or together
Splash lubrication
Splash lubrication is achieved by partly submerging the crankshaft in the oil sump. The momentum of the rotating crankshaft, splashes the oil to the other engine components such as cams lobes, wrist pins and cylinder wall.
Forced lubrication
Pressurized lubrication uses an oil pump to provide pressure rise film of lubricant between the moving parts such as main bearing rod bearing and cam bearings. It also comes old to the engine valve guides and Rocker arms.