What is "Viscosity Index" and when is it important?

Viscosity Index can be described as the rate by which viscosity changes with the change in temperature. Viscosity index is important when you are operating at high or very low temperatures. High Viscosity Index lubricants are also important when you are operating over a very wide temperature range. i.e. a pump which starts up at very cold temperatures but the process material being pumped is at a very high temperature resulting in the lubricating oil going from a very low temperature to a very high temperature.