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Motor oil, or engine oil, is an oil used for lubrication of various internal combustion engines. While the main function is to lubricate moving parts, motor oil also cleans, inhibits corrosion, improves sealing, and cools the engine by carrying heat away from moving parts.[1]


Motor oils are derived from petroleum-based and non-petroleum-synthesized chemical compounds. Motor oils are today mainly blended by using base oils composed of hydrocarbons (mineral, polyalphaolefins (PAO), polyinternal olefins[2] (PIO), thus organic compounds consisting entirely of carbon and hydrogen. The base oils of some high-performance motor oils contain up to 20 wt.-% of esters.[3]


In engines, there are parts which move against each other causing friction which wastes otherwise useful power by converting the energy to heat. Contact between moving surfaces also wears away those parts, which could lead to lower efficiency and degradation of the engine. This increases fuel consumption and decreases power output and can, in extreme cases, lead to engine failure.

Lubricating oil creates a separating film between surfaces of adjacent moving parts to minimize 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.

Coating metal parts with oil also keeps them from being exposed to oxygen, inhibiting oxidation at elevated operating temperatures preventing rust or corrosion. Corrosion inhibitors may also be added to the motor oil. Many motor oils also have detergents and dispersants added to help keep the engine clean and minimize oil sludge build-up.

Rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against moving parts, causing wear. Because particles accumulate in the oil, it is typically circulated through an oil filter to remove harmful particles. An oil pump, a vane or gear pump powered by the engine, pumps the oil throughout the engine, including the oil filter. Oil filters can be a full flow or bypass type.

Internal combustion engines[edit]

Four-stroke engine or Otto cycle internal combustion engines, which include gasoline and diesel Reciprocating engines and Wankel engines, use multi-viscosity or single-viscosity oils depending on the engine application.

Two-cycle internal combustion engines use a special motor oil designed to be mixed with the fuel and burned. This oil provides the same lubricating properties as oil used in four-cycle engines. This type of oil is either pre-mixed with the fuel in the tank, or injected by the engine into the fuel-air mixture. Non-smoking 2-cycle oils are composed of esters or polyglycols. Environmental legislation for leisure marine applications, especially in Europe, enhanced the use of ester-based two cycle oils.

Other oils[edit]

Other types of oil used in automotive applications include two-cycle oil for use in two-cycle engines, Automatic Transmission Fluid (ATF) and gear oil that are used in manual transmissions, transfer cases and differentials.

  1. ^ Klamman, Dieter, Lubricants and Related Products, Verlag Chemie, 1984, ISBN 0-89573-177-0
  2. ^ G. Corsico, L. Mattei, A. Roselli and C. Gommellini, Poly(internal olefins)- Synthetic Lubricants and high-performance functional fluids,, Marcel Dekker, 1999,Chapter 2, p. 53-62, ISBN 0-8247-0194-1
  3. ^ R.H. Schlosberg, J.W. Chu, G.A. Knudsen, E.N. Suciu and H.S. Aldrich, High stability esters for synthetic lubricant applications, Lubrication Engineering, February 2001, p. 21-26