INS AND OUTS OF TRANSMISSION FLUID
Many people can go the entirety of their ownership over their vehicle without ever changing the transmission fluid. Although some vehicles market a lifetime fluid on the transmission fluid and will advise against changing it so that the vehicle does not begin to slip while changing gears, experts would still highly recommend otherwise.
WHEN TO REPLACE
Changing your transmission fluid is advisable every 50,000 miles on average to maintain the vehicle in the best running conditions possible. Automatic vehicles use a torque converter to shift gears, and the torque converter and the transmission are not physically connected so the fluid is what allows the movement to continue between them. The fluid allows the mechanical spinning of the engine to be transferred to the transmission using fluid pressure and fluid friction. You need clean fluid to assure that there is no unwanted build-up or friction in this process.
Maintaining the transmission fluid clean is the most important aspect of this process, if you go 150,000 miles without changing the transmission fluid when your vehicle has become accustomed to running on that texture of fluid if you were to change the transmission fluid at that point it could potentially cause the vehicle to slip. If the transmission fluid is black then the converter has been functioning on dirty transmission fluid, which tends to have more friction. If you were to change the fluid at this point, you would be taking dirty fluid and adding clean fluid, which would likely cause the converter to begin to slip. At this point changing the transmission fluid could potentially be more problematic than helpful. The trick is to always have it well maintained. In this case, it might even be better to leave the transmission fluid alone if the transmission is still working.
Preventative maintenance is key here, a lot of the time if you wait too long and the transmission fluid is too dirty on a high mileage vehicle then changing the transmission fluid might cause the transmission to have more problems and in some cases might cause the transmission to stop working properly altogether.
IMPORTANT POINTS TO REMEMBER:
Change the transmission fluid regularly on average every 50,000 miles or so
On high mileage vehicles with dirty transmission fluid do not change for clean fluid
Preventative maintenance is key
Changing dirty fluid for clean fluid can cause the converter to begin slipping
Dirty fluid has more friction and transmission running on dirty fluid will slip if transferred to clean fluid
Even if there is a lifetime fluid guarantee expert recommend still maintaining your transmission fluid clean with maintenance
Just to recap why transmission fluid is essential and how it affects your vehicle. In an automatic vehicle, the transmission uses the torque convertor to shift gears and although the torque converter is bolted to the flywheel on the engine the connection between the torque converter and the transmission is using fluid dynamics to make it go, it is not physically connected as it is in a manual vehicle. Always stay on top of your preventative maintenance and avoid costly repairs for yourself in the future!