Transmission fluid is a liquid that acts as a lubricant for parts inside transmission. In an automatic transmission, this fluid also works as a coolant and a viscous fluid that transmits power from the engine to the transmission.
Most manufacturers recommend that manual transmission fluid be changed every 40,000 to 50,000 miles and automatic transmission fluid - every 60,000 to 80,000 miles. A variety of fluids are used for different transmissions.
Bearings and gears in the manual transmission wear out in time. As a result fluid contamination occurs over time. These contaminants shorten the life of your transmission. More heat generated in an automatic transmission is the reason of fluid degradation, in addition to contamination. If you don't change the transmission fluid on schedule, this will shorten the transmission's life.