American Petroleum Institute (API) categories efficiency levels of engine oils into two general classes. Petrol engine oils are represented by “S” (i.e. service station) and gasoline engine oils are shown by letter C (i.e. commercial). The efficiency level of every oil belonging to these two groups is shown by alphabetic letters after S or C letter.

This notion refers to the resistance of oil against flowing. It results from internal frictions of molecules. By changing oil temperature, its viscosity will change. Warmer oil signifies lower viscosity. Usually, viscosity is measured at 40 and 100°C.

-Transferring generated heat in engine parts -Absorbing shock of engine parts and bearings -Sealing of piston rings and body of pistons -Transferring soot and external materials; preventing particles from deposition -Preventing from rust and corrosion of internal engine parts -Resistance against oxidation -Neutralization of acids