quote:An arbitrary scale used to show the magnitude of viscosity changes in lubricating oils with changes in temperature. Oils with low 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 D 2270 are widely used to determine VI number.
However, VI does not tell the whole story -- it only reflects the viscosity/temperature relationship between temperatures of 40°C and 100°C. Two lubricants or base oils with the same VI number may perform dramatically different at low temperatures in the -5°C to - 50°C range
Well, sometimes a high viscosity index is due to a lot of VII additive, in which case I am not sure the higher is the better. But if a high viscosity index is due to use of a quality base oil, like ester, then the higher the better for sure.