This chapter reviews the lubrication requirements of high-speed diesel and petrol engines, and medium-speed trunk piston diesel engines, and the oil properties required for their satisfactory lubrication. The lubrication problems in petrol engines are very much the same as those found in diesel engines, but are usually less severe and emphasise different aspects. The lubrication of the diesel engine power assembly (piston, piston rings and cylinder liner) is a more severe problem than the lubrication of its valve-actuating components. Engine lubrication systems are normally pressure-fed from an oil pump driven by the engine itself. However, some parts of the engine may be lubricated by gravity or splash from the action of the moving parts that dip into the sump oil and cause it to be flung to various internal engine parts. The pressure-fed system is used in the main to lubricate the bearings of the engine and valve-actuating component.