Fluids used in manual transmissions of automotive engines must be formulated to perform many critical functions. These include good synchromesh performance, easy shifting at low temperature, protection against corrosion, high load carrying capacity, low friction, good resistance to pitting, good thermal and oxidative stability and elastomer compatibility.Viscosity is one of the critical properties of a gear lubricant. Transmission oils must be sufficiently fluid at low temperature in order to minimize energy losses due to friction and to provide easy gear shifting. On the other hand, the viscosity of the fluid must be high enough at high temperature to avoid excessive wear. The operating temperature for gear lubricants keeps on increasing as a result of higher engine output, increased vehicle loading and improved aerodynamic body styling that reduces airflow around transmission. Therefore, an important objective in formulating gear lubricants is to minimize the operating temperature of the oil through the selection of proper gear additives and base stocks.Multigrade gear lubricants containing highly shear stable VI Improvers are often used to formulate fill for life gear lubricants providing the viscosity profile needed to operate over a wide range of temperature. The selection of the VI Improver must be based on its contribution to viscosity at low and high temperature, its shear stability, its thermal and oxidative stability but also, as indicated earlier, on its ability to minimize the operating temperature of the gear lubricant.We have used the Axial Groove-Ball-Bearing Test (ARKL) developed by VW to evaluate the effect of the base oil viscosity and type and of the VI Improver type on the operating temperature of gear oils.