Wikipedia
Viscosity index (VI) is an arbitrary measure for the change of viscosity with variations in temperature. The lower the VI, the greater the change of viscosity of the oil with temperature and vice versa. It is used to characterize viscosity changes with relation to temperature in lubricating oil.
The viscosity of liquids decreases as temperature increases. The viscosity of a lubricant is closely related to its ability to reduce friction. Generally, the least viscous lubricant which still forces the two moving surfaces apart is desired. If the lubricant is too viscous, it will require a large amount of energy to move (as in honey); if it is too thin, the surfaces will come in contact and friction will increase.
Many lubricant applications require the lubricant to perform across a wide range of conditions, for example, automotive lubricants are required to reduce friction between engine components when the engine is started from cold (relative to the engine's operating temperatures) up to when it is running. The best oils with the highest VI will remain stable and not vary much in viscosity over the temperature range. This allows for consistent engine performance within the normal working conditions.
The VI scale was set up by the Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are . The original scale only stretched between VI=0 (lowest VI oil, naphthenic) and VI=100 (best oil, paraffinnic) but since the conception of the scale better oils have also been produced, leading to VIs greater than 100 (see below).
Viscosity Index
Classification
..35
Low
35..80
Medium
80..110
High
110..
Very High
VI improving 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.
The viscosity index can be calculated using the following formula:
$V = 100 \cfrac{{(L-U)}}{{(L-H)}}$
where V indicates the viscosity index, U the oil's kinematic viscosity at , and L & H are values based on the oil's kinematic viscosity at . L and H are the values of viscosity at 40°C for oils of VI 0 and 100 respectively, having the same viscosity at 100°C as the oil whose VI we are trying to determine. These L and H values can be found in ASTM D2270.