What Hurts a Transmission
The main factor in transmission failure is heat. All vehicles are equipped with a radiator cooling system to keep their engine cool. Inside the radiator is a separate compartment to cool down the transmission oil. If this fails or partially gets blocked, overheating of the transmission can occur therefore making chemical breakdown and changes the viscosity of the transmission fluid. As you can see in this chart, small increments of higher heat can drastically change the longevity of the vehicle's transmission. Regular transmission fluid exchanges are recommended every 40,000 miles for most manufacturers.