A transmission service is part of your routine maintenance, just like replacing your engine oil.
A complete transmission service should include:
• removing and examining the sump or pan (where possible)
• replacing or cleaning the screen or filter
• cleaning the pan
• reinstalling the pan with a new pan gasket
• pumping out the rest of the old fluid and replacing it with new, high quality ATF
• adding a friction modifier or additive package (model specific)
New fluid restores the holding power between the clutches, reducing slip and heat production.
At the same time, flushing the old fluid washes away tiny particles of clutch material and metal shavings. Those particles clog passages and wedge between moving parts, causing wear throughout the transmission. Changing the fluid eliminates those particles and the wear they cause.