Lubricants for gasoline and diesel engine are different as the load, cycles and fuels are different. The oil in an engine does more than just reduce friction between its moving parts. Combustion of rich air-fuel mixture during starting, idling and warm up form deposits. These deposits in the form of varnish, sludge, soot and carbon, interfere with proper engine operation. The engine oil keeps all the deposit forming material in suspension and gets rid of them by oil filter, or draining out at proper intervals.
Engine oil is stored in the oil pan or sump at the bottom of the engine.
A pump forces the oil through a filter and then through a series of passages and galleries to lubricate the engine's moving parts. The flow of also cools these parts. Rapidly moving engine parts actually float on a thin film of oil and never make contact with one another. This is called hydrodynamic lubrication and usually begins when an engine reaches the idle speed. Most engine wear occurs when a cold engine is first started, before the oil reaches its normal operating pressure and flow.