Engine Oil and its viscosity

Engine Oil and its viscosity: - For satisfactory lubrication of the engine the oil should possess some functional properties of which viscosity of oil is one of the most important properties, as it brings out the oil’s capacity to lubricate. Viscosity is the measure of oil’s resistance to flow. An oil’s viscosity is identified by its SAE’s (Society for Automotive Engineer’s) number. The thinner and oil, lower its number, e.g. SAE 10 W. The numerical relates to viscosity at particular temperature and the alphabet ‘W’, indicates the oil’s suitability for colder temperature. With the viscosity index improver, the viscosity increses at higher temperature and at lower temperature it does not increase significantly, thus achieving optimum viscosity at lower and higher temperature. Such oils are called multigrade oils, for instance ‘20 W40’ shows thinness at low temperature and thickness at higher temperature.

However, there are other service classification of oil apart from viscosity, developed by API- American Petroleum Institute, which indicates service characteristics. It is graded on a scale from SA (the lowest) to SJ (the highest) for gasoline engines, it is graded on a scale from CA to CG. Both the recommendations for viscosity and service classification can be found on label of the oil container.