When should automatic transmission fluid be replaced?

Transmission fluid or transmission oil is used to lubricate and cool a transmission. In automatic transmissions, it is also used in the torque converter to transfer power from the engine.

Replacing transmission oil involves removing a worn out one and replacing with a fresh and same or compatible oil.

Automatic transmission of a car is very delicate and should be taken care of.

It is very important to carry out oil change preventive maintenance on a car's transmission to ensure its longevity.

Often, car users don't know when the transmission oil is due for change. This happens especially when the car was acquired as a used car and the mileage at which previous oil change was done is unknown. Furthermore, some manufacturers do not give any service mileage for transmission oil but state that there is "lifetime" fluid which does not require maintenance in the transmission.
Users reviews have shown that some people have had troubles with their transmissions because they followed this recommendation and didn't change the ATF (Automatic transmission fluid).
The word "lifetime" does not imply the whole life time of operation of the car but rather around 100,000 miles. Cars are actually not meant to last forever.

BELOW ARE HINTS ON HOW TO KNOW WHETHER THE TRANSMISSION OIL IS DUE FOR REPLACEMENT:

The oil is dark: Over time, particles from clutch materials and microscopic metals will be suspended in the transmission fluid which will give it a dark coloration after a long period of extended usage.

Gear slipping - Wrong fluid, degraded fluid or low fluid level can cause gears to slip.

Rough shift - When the fluid becomes dirty, dirts find its way into parts of the valve body and a dirty fluid will also clog the transmission oil filter over time. This will reduce the flow and pressure of the fluid in the transmission.

Delayed engagement: Delayed engagement is when the transmission does not engage gears immediately you shift into drive or reverse positions. A worn out fluid can cause a delay in shift.

Color: There are different colors for transmission fluids, the popular color being red. There is green and other colors. When the oil is new, it is usually clear (transparent) but as it ages, it gradually turns from clear to opaque to dark. The oil should be changed before it gets dark.

Grinding noise: Whenever you hear grinding noise or other unusual noises from the transmission, it's good to check both fluid level and quality.

Mileage: Some manufacturers recommend the mileage at which the transmission fluid should be replaced. This recommendation should strictly be adhered to.

Leak: When transmission leak is noticed, it is good to check both fluid level and quality. It is important to also check fluid quality because when the fluid level is low, it can overheat causing the fluid to be degraded.

Driving condition: For severe driving conditions like towing and carrying heavy weights especially weights that are above the specified maximum capacity of the car, harsh driving (severe acceleration and constant high speed driving) and severe off-road conditions, the transmission fluid may be replaced earlier than recommended by the manufacturer.