The transmission fluid should be replaced on a regular basis. Regardless of having a manual or automatic transmission fluid, regularly changing it will avoid costly transmission repairs.
Transmission fluid is a greasy lubricant for all moving parts inside a vehicle's transmission. In an automatic transmission, the fluid acts as a coolant, transmitting power from the engine to the transmission. Manual transmissions use different fluids, such as oils, motor oils and hypoid gear oils.
Most car manufacturers recommend a change in manual transmission fluid every 30,000-60,000 miles. For automatic transmission fluid, some car makers suggest a change every 40,000 miles. For practical purposes and to avoid expensive repair costs, the transmission fluid should be replaced regularly.