Viscosity index

Viscosity index is a petroleum industry term. It is a lubricating oil quality indicator, an arbitrary measure for the change of kinematic viscosity with temperature. The viscosity of liquids decreases as temperature increases. The viscosity of a lubricant is closely related to its ability to reduce friction. Generally, you want the thinnest liquid/oil which still forces the two moving surfaces apart. If the lubricant is too thick, it will require a lot of energy to move the surfaces (such as in honey); if it is too thin, the surfaces will rub and friction will increase.

As stated above, the Viscosity Index highlights how a lubricant's viscosity changes with variations in temperature. Many lubricant applications require the lubricant to perform across a wide range of conditions: for example, in an engine. Automotive lubricants must reduce friction between engine components when it is started from cold (relative to engine operating temperatures) as well as when it is running (up to 200 °C). The best oils (with the highest VI) will not vary much in viscosity over such a temperature range and therefore will perform well throughout.

The VI scale was set up by the Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are 100 °Fahrenheit (40 °C) and 210 °F (100 °C). The original scale only stretched between VI=0 (worst oil) and VI=100 (best oil) but since the conception of the scale better oils have also been produced, leading to VIs greater than 100 (see below).

VI improver additives and higher quality base oils are widely used nowadays which increase the VIs attainable beyond the value of 100. The Viscosity Index of synthetic oils ranges from 80 to over 400.