There's nothing more important for your engine than oil and the same applies to your transmission. Gear oil, or hydraulic fluid, is the lifeblood of your transmission, whether automatic or manual. No matter what type of vehicle you drive, from a small economy car to a large SUV, the transmission would be unable to function were it not for the vital performance of the fluid within it.
Generally speaking, there are two major types of gear oil. True gear oil has a much greater viscosity than hydraulic fluid, though gear oil is often the named applied to both types of oil. Gear oil is used in many other applications than simply transmission. Many automotive manufacturers have developed specialized gear oil formulations for better differential operation. Gear oil is also used in professional landscaping equipment and many forms of heavy machinery such as bulldozers and front-end loaders.
Gear Oil: Gear oil used in manual transmissions is usually much thicker and has a higher viscosity than that found within automatic transmissions. Manual transmissions do not employ hydraulic pumps to distribute the fluid throughout the engine, rather, only the lower gears are submerged or partially submerged within the oil and it is the contact between the gears that distributes the gear oil throughout the transmission. The gear oil found within manual transmissions is often a yellow or golden color, while most automatic transmission fluids are red.
Automatic Transmission Gear Oil: This special type of gear oil is a combination of modern technology and innovation. It is much thinner and lighter than conventional gear oil, in addition to the different coloration, though it provides excellent protection to the vital components within the transmission housing. Automatic transmission fluid is not recommended for use in other applications, though some people will pour a small amount within the crankcase of a vehicle to help stop leaking seals. This is a short-term solution for the problem, however, and should not be used in place of repair. Automatic transmission fluid is also not suitable for use with manual transmissions or in differentials.
The Ratings: Like all types of oil, gear oil comes labeled with the weight of the fluid, or the viscosity rating versus temperatures. 75W90 or 80W90 is often the recommended weight for differential purposes, though the vehicles manual should be checked to determine whether there are any required additives or specially made gear oil that must be used for the application. The ratings can be somewhat misleading, as they apply strictly to that type lubricant. For example, gear oil with the rating of 75W90 has a viscosity similar to standard 10W40 motor oil, though there are obviously many differences within the physical structure of the oil itself.
Differentials: While differentials may use the same weight gear oil, some have begun to require a different GL rating. Often, a transmission will require GL4 rated fluid, while the differential will require GL5 rated oil. This simply means that the gear oil has been specially formulated for use within a differential and that it contains different additives and performance enhancers than transmission fluid.