This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
Check your service manual. The severe service definition means that
the vehicle is operated *primarily* in one of those conditions.
Mopar 3-speed automatic transmissions need to have oil
and filter changed when the oil gets discolored due to suspended
solids. ATF usually does not need to be changed unless
contaminated. The trans oil can get contaminated by overheating or
by severe internal wear due to abuse, especially towing.
(Mostly from Robert Muir).
4-speed automatic transmissions should have their ATF
changed every three years or so. It MUST be replaced
with ATF+4, NOT Dexron. (Newer automatics require ATF+5).
MODERN CHRYSLER TRANSMISSIONS WILL BE DESTROYED
IF YOU USE THE WRONG FLUID.
Even some 3-speed DC transmissions are NOT compatible
with Dexron - read your manual !!! Or just use ATF+4
(ATF+5 for some newer models)