Engine oil or Motor oil is a libricant used for lubrication of various internal combustion engine parts. The four functions of engine oil are to lubricate moving parts; clean, inhibit corrosion, improve sealing, and dissipating heat generated in the engine and keep moving parts cool.

An automotive engine, is composed of several moving parts which are in constant contact with other parts causing friction which wastes otherwise useful power by converting the energy to heat. Contact between moving surfaces also wears away components such as the main and connecting rod bearings, the cylinder liners etc, which could lead to lower efficiency and wear of the engine. This inturn will increases fuel consumption, decrease power output and can, in extreme cases lead to engine seizure. Lubricating oil acts as a barrier between surfaces of adjacent moving parts minimizing direct contact between them, decreasing heat caused by friction and reducing wear, thus protecting the engine. In use, motor oil transfers heat through convection as it flows through the engine by means of air flow over the surface of the oil pan, an oil cooler and through the build up of oil gases evacuated by the Positive Crankcase Ventilation (PCV) system. In gasoline engines, the top most piston ring can expose the motor oil to temperatures of 320 °F.

The extreme temperatures eventually destroy the properties of the oil and it will no longer be capable of carrying out it's functions. The engine oil has a detergent quality which absorbs the carbon formed inside the engine and a certain amount of oil can assimilate a certain amount of carbon after which it cannot. The only way to get the best out of the lubricating system is to replace the engine oil and filter.