Gear Oil

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").

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 synchros can not 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.