The transmission and differential are the workhorses of your vehicle. They take the power your engine makes and turn it into forward motion.
Like all bearings and gears, the ones in your drive train rely on clean lubricant to work properly and live a long time. As they wear, tiny metal particles get washed away and are suspended in the oil. Unlike an engine, there is no filter to trap these contaminants. The only way to get rid of the contaminants is by changing the drive train fluids.
All manual transmissions need this service, along with most 4-wheel drive, and rear-wheel drive automatic transmission vehicles. It all depends how the transmission and differential are put together.
How often you change the drive train fluids depends on the kind of driving you do with your vehicle:
Happy Motoring,
Amy