What is Oil Viscosity or Oil Weight? 

Viscosity is a fluid’s resistance to flow. In case of lubricants the viscosity is very important because it affects the oil’s ability to reduce friction and transfer heat. The viscosity is measured in mPa*s (millipascal-seconds) or its equivalent cP (centiPoise) but in everyday life we don’t use the actual measured viscosity, we use viscosity grades instead. In case of engine oils, these grades, also known as “weights,” were set by the SAE (Society of Automotive Engineers) and in order for a fluid to fall in a given category it has to meet certain limits. These limits are listed in the SAE J 300 table: 

The numbers that have a W after them are the so-called winter viscosity grades while the numbers that don’t have a W are the so-called summer- or operating temperature viscosity grades. Several decades ago, motor oils either fell in one of the winter viscosity grades or in one of the summer viscosity grades. These products were monograde oils and car owners had to change oil at least twice a year: once for the winter and once for the summer. Thanks to the synthetic base oils and the viscosity modifier additives, nowadays most motor oils are so-called multigrade oils that don’t change their viscosity with temperature that much, so they meet the limits of a winter and that of a summer viscosity grade at the same time. These oils can be used regardless of season. Monograde oils are still used for special purposes but usually not as engine lubricants.