Ans-: The rate at which the viscosity of an oil changes with temperature is measured by an empirical number, known as the Viscosity-Index (V.I.). A relatively small change in viscosity with temperature is indicated by high viscosity index. Whereas a low viscosity index shows a relatively large change in viscosity with temperature. The pennsylvanian oils, consisting mainly of paraffins, are arbitrarily assigned a viscosity index value of 100 as they exhibit a relatively small change in viscosity with a rise in temperature. Oils of gulf-coast origin, consist mainly of alicyclics (napthenes), are arbitrarily assigned a viscosity index value of 0 as they exhibit a larger change in viscosity with a rise in temperature. In industry, lubricating oils of high viscosity index are preferred since they have practically the same viscosity over a range of temperature.
