Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It has high viscosity and usually contains organosulfur compounds. Some modern automatic transaxles (integrated transmission and differential) do not use a heavy oil at all but lubricate with the lower-viscosity hydraulic fluid, which is available at pressure within the automatic transmission. Gear oils account for about 20% of the lubricant market.
Most lubricants for manual gearboxes and differentials contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears. Typical additives include dithiocarbamate derivatives and sulfur-treated organic compounds ("sulfurized hydrocarbons").
thumbnail|right|Gear oil being added to the final reduction gears in a scooter.
EP additives which contain phosphorus/sulfur compounds are corrosive to yellow metals such as the copper and/or brass used in bushings and synchronizers, unless properly buffered; the GL-1 class of gear oils does not contain any EP additives and thus used to be the choice in applications which contain parts made of yellow metals.
GL-5 is not necessarily backward-compatible in synchro-mesh transmissions which are designed for a GL-4 oil: GL-5 has a lower coefficient of friction due to the higher concentration of EP additives over GL-4, and thus such transmissions running on it cannot engage as effectively, unless a specialized friction modifier has been included within the oil's additive package; synchro-mesh compatibility is usually explicitly stated and such oils are often known in the trade as TDL (Total Drive Line) oils.
API ratings
Gearbox oils are classified by the American Petroleum Institute using GL ratings. The higher an oil's GL-rating, the more pressure can be sustained without any metal-to-metal contact taking place between transmission components. Separate differentials usually have higher pressure between metal parts than gearboxes and therefore need higher GL-rating. For example, most modern gearboxes require a GL-4 oil, and separate differentials (where fitted) require a GL-5 oil.
While they take the same form, the viscosity grades for gear oils are on a different scale than the viscosity grades for an engine oil. The viscometrics for gear oils are standardized in SAE J306. Multigrade gear oils are becoming more common; while gear oil does not reach the temperatures of motor oil, it does warm up appreciably as the car is driven, due mostly to shear friction (with a small amount of heat conduction through the bellhousing from the engine block).
Fully synthetic gear oils are also used in many vehicles, and have a greater resistance to shear breakdown than mineral oils.
API Category GL-1 (inactive) designates the type of service characteristic of manual transmissions operating under such mild conditions of low unit pressures and minimum sliding velocities, that untreated oil may be used satisfactorily. Oxidation and rust inhibitors, defoamers and pour depressants may be used to improve the characteristics of lubricants intended for this service. Friction modifiers and extreme pressure additives shall not be used.
API Category GL-2 (inactive