What is the Viscosity Index (VI) and what does it mean?

Many people have probably come across the concept of viscosity index when buying oil or grease, e.g. for a motorcycle. The seller or maybe someone on the forum mentioned that the higher the index, the better, but... what does it mean in practice and what impact does it have on a specific application?

The viscosity index (VI) is a measure of how the viscosity of an oil or grease changes with temperature. The higher the temperature, the lower the viscosity of the oil or grease and the more fluid it becomes; the lower the temperature, the higher the viscosity and the thicker the grease or oil becomes. Changes in the viscosity of grease and oil under temperature have a real impact on their performance.

Oils and greases with a high viscosity index shows more moderate and slower changes in viscosity and density under changing temperature, and therefore have more stable operational properties than oils and greases with a lower VI. This reduces the frequency of oil or grease changes, thus saving time and costs associated with machine maintenance. To sum up, viscosity index is an important parameter to consider when selecting an oil or grease to ensure optimal performance of equipment under varying operating conditions.