Viscosity index or VI is measure of the temperature at which a lubricating oil works properly. The VI scale goes from 0 to 100, with 0 being the worst, and 100 being the best. New products are now better than when the scale was first made, so some new products have scores as high as 400.
Lubricating oil is meant to reduce friction between two surfaces. If it is too viscous the oil will be thick and sticky and need too much energy to keep the parts moving. If the oil is too thin, then the parts will rub together and create friction. What is needed is an oil that has just the right amount of viscosity to keep the two surfaces apart. Oil becomes less viscous as the temperature goes up. In an automobile engine, the oil needs to be viscous at a range of temperatures. It needs to work when the engine is first started and is cold, and keep working when the engine gets very hot, perhaps as much as 200 °C. The best oils, with a high viscosity index score, will keep their viscosity the same across the temperature range.
The scale was made up by the Society of Automotive Engineers (SAE).