High temperatures have a thinning effect on oil viscosity.  As a result, the ability of a gear oil to support a load decreases as temperatures increase.  The rate of viscosity change is different for each oil, with the rate of change expressed as the Viscosity Inde (VI) number.

The higher a lubricant's VI number, the less effect heat has on its viscosity.  According to Noria Corporation, "If you have a gear box running at high-temperature that seems to have a low reliability rating, check to see if the oil has suitable VI, and is capable of holding up under the high temperatures.  If the product's VI is less than 120, as shown on the product data sheet, then consider an alternative oil for the high-temperature, heavily loaded application."

AMSOIL Synthetic gear lubes all feature VI numbers over 120, providing superior protection in high temperature applications.