The determination of the viscosity index is based on two oil types with the same starting viscosity and strongly diverging changes in viscosity as temperature rises. The oil showing a strong temperature dependence is assigned the viscosity index 0, whereas the oil with a lower change in viscosity is assigned the viscosity index 100. The viscosity index according to DIN 2909 is calculated by comparing the change in viscosity of an oil with the two oils as a reference.