Gear oil is found in transmissions, differentials and other types of gearboxes in automobiles and other machines. Its purpose is to protect and lubricate the gears it surrounds. Gear oil differs from engine oil mainly because of its higher viscosity. The Society of Automotive Engineers (SAE) publishes a viscosity grading guide for gear oil, which is a different grading scale than the SAE’s engine oil scale.

Monograde Gear Oil

The SAE rates gear oils suitable for operation at one temperature on a monograde scale. Gear oil meant for high ambient temperatures is designated by a single number (for instance, SAE 80 or SAE 250). Viscosity of these oils is measured at 212 degrees Fahrenheit; higher numbers indicate more viscous oil. Monograde gear oil with a “W” after its number (SAE 70W, SAE 80W) has a viscosity rating measured at 0 degrees Fahrenheit and is intended for low ambient temperatures.