Transmission fluid is the liquid that is used to provide lubrication to the moving parts in the transmission system of an engine; this is achieved because of its slippery nature. This fluid also plays a role in the cooling down of the engine as it is a very good conductor of heat.
In manual transmission vehicles, you can use ordinary engine oil, heavy duty gear oil or even automatic transmission fluid; in an automatic transmission engine it is called automatic transmission fluid. This fluid should not be confused with engine oil as it is red in color. Various models of cars require different types of ATF for the best protection and performance. Toyota vehicles from 1991 use two types of automatic transmission fluid: the Dexron III, also known as Mercon, and the ATF Type T-IV.
This is a multipurpose ATF useful in various types of transmission and is approved by various car manufacturers. Dexron III is mixed with refined base stocks, has super oxidation stability plus resistance to corrosion, rusting and foaming. It can withstand high temperatures of up to about 300 degrees Fahrenheit, thus allowing smooth shifting even in high temperatures. The blending of this ATF with friction additives allows it smooth shifting at the same time attaining positive lock up. The high viscosity index helps maintain adequate lubricating film thickness in high temperatures, and also remarkable fluidity at low temperatures. Dexron III lasts longer because of the refined base oil and oxidation inhibitors in it that slow down the rate of the fluid breakdown at high temperatures.
This type exhibits almost similar characteristics as the Dexron III and can also be used for 1991 Toyotas. Its use was tested and approved by Toyota. It prevents shudder thanks to its accurate coefficient of friction. It also has additives to help protect the engine from extra wear and corrosion. Smooth shifting is achieved even at high temperature. It is made up of chemicals that allow usage with rubber seals and metal finishes. Only use this type of automatic transmission fluid if it is advised in the vehicle's user manual. Change fluid every 30,000 miles; however, regular checks should be done and if the fluid smells like toast and is rust colored, replace immediately before further damage to the engine.
