Automatic transmission oil helps preserve seals and gaskets, lubricate bushings, bearings and gears and cools the transmission. The specifications of an automatic transmission oil will determine the temperature it can withstand, its oxidation stability and its anti-corrosion capabilities. Automatic transmission oil must be changed regularly --generally every two to three years, or every 24,000 to 36,000 miles. Use the transmission fluid recommended by your vehicle's manufacturer. If you buy an aftermarket high-performance fluid, make sure it is compatible with your vehicle's model.
The pour point of a transmission oil is the temperature at which the oil no longer flows in a test jar. This occurs at extreme low temperatures and is only an issue in very cold regions. The pour point of automatic transmission fluids will vary from -9 degrees Celsius -- for the Ama-Tran Powershift Fluid 50 -- to -51 C for the Mercon V Synthetic Blend 0W-20.
The viscosity rating of a transmission oil determines the thickness and ability to flow at high temperatures and low temperatures. The higher the viscosity index of an oil, the less its viscosity varies over temperature, which makes it a more reliable lubricant. For instance, the viscosity index of Esso LT 71141 is 168 while the viscosity index of AMSOIL synthethic automatic transmission fluid is 153.
You can also choose your automatic transmission oil by selecting the ATF type your manufacturer recommends. ATF oils are often introduced by car manufacturers, although outside manufacturers can make any AT fluid type as long as it meet the specs set forth by the automaker. For instance the Type F automatic transmission oil was launched by Ford and is now also used by Toyota. The type H was also launched by Ford in 1987 and can be replaced by the Mercon and Mercon V. GM vehicles favor the Dexron -- Dexron I to Dexron VI depending on the model -- for their automatic transmissions. Honda, on the other hand, uses its proprietary Genuine Honda ZL ATF.
The flash point of an automatic transmission oil describes the temperature at which the oil will produce enough vapor to ignite if a flame is applied. Automatic transmission oils must withstand high temperatures, so the higher the flash point of an oil the better. For example, the flash point of AMSOIL, a synthetic automatic transmission fluid, is 234 C while ESSO LT 71141, a semi-synthetic automatic transmission fluid has a flash point of 215 C.
