Grades Of Motor Oil

The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. For singlegrade oils, the kinematic viscosity is measured at a reference temperature of 100 °C 212 °F in units of mm²/s or the equivalent older nonSI units, centistokes abbreviated cSt. Based on the range of viscosity the oil falls in at that temperature, the oil is graded as an SAE number 0, 5, 10, 20, 30, 40, 50, 60 or 70. The higher the viscosity, the higher the SAE grade number is. These numbers are often referred to as the weight of a motor oil. The reference temperature is meant to approximate the operating temperature to which motor oil is exposed in an engine.
The viscosity of singlegrade oil derived from petroleum unimproved with additives changes considerably with temperature. As the temperature increases, the viscosity of the oil decreases logarithmically in a relatively predictable manner. On singlegrade oils, viscosity testing can be done at cold, winter W temperature as well as checking minimum viscosity at 100 °C or 212 °F to grade an oil as SAE number 0W, 5W, 10W, 15W, 20W, or 25W. A singlegrade oil graded at the hot temperature is expected to test into the corresponding grade at the winter temperature i.e. a 10 grade oil should correspond to a 10W oil. For some applications, such as when the temperature ranges in use are not very wide, singlegrade motor oil is satisfactory for example, lawn mower engines.