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 100 degrees Celsius; 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 0-17.778 degrees Celsius and is intended for low ambient temperatures.
Multigrade Gear Oil
Some gear oils have additives that change their viscosity at different operating temperatures. These gear oils are labelled "multigrade" by the SAE, which provides a rating for the oil's low- and high-temperature viscosity. For instance, SAE 80W-90 gear oil has a low-temperature rating of 80 and a high-temperature rating of 90.
Comparing Oil Viscosity
As mentioned previously, gear oil viscosity numbers are not directly comparable to engine oil viscosity numbers. For example, 75W-90 gear oil is about the same viscosity as 10W-40 engine oil; 80W-90 is about the same as 20W-40. (See Resources for a comparison chart.)