An arbitrary scale used to show the magnitude of viscosity changes in lubricating oils with changes in temperature. Oils with low viscosity index(VI) number such as VI=0(zero) have high dependence of viscosity change on temperature. They thicken quickly with decreasing temperature, and thin out quickly with increasing temperature. Oils with high VI number such as VI=200, will still thicken with decreasing temperature but not as rapidly, and also will thin out with increasing temperature, but again not as much as low VI oil.
VI number can also be "negative".
Tables found in ASTM Method D2270 are widely used to determent the VI number.
However, VI does not tell the whole story --- it only reflects the viscosity/temperature relationship between temperature of 40℃ and 100℃. Two lubricants or base oils with the same VI number may perform dramatically different at low temperatures in the -5℃ to -50℃ range.
