Viscosity is the measure of oil's resistance to flow.
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. American Petroleum Institute, which indicates service characteristics. Both the recommendations for viscosity and service classification can be found on label of the oil container.