The viscosity index is based on measured viscosities at 40??? C. and at 100??? C. The viscosity index of oils containing sufficient wax to render the measurement of a viscosity at 40??? C. difficult or impossible may be determined by an extrapolation method, such as by measuring the viscosity of the oil at two temperatures at which the oil is fluid, e.g. 70??? C. and 100??? C., with the viscosity