You know this thread is from almost 4 years ago, right?
Originally Posted by RacerX
-- Why can't you use Dex IV? - says who? Is there a study, or someone tested it???
Isn't it supposed to be even better than III?
Also -- is the cheap store brand just as good (Dex III)
ATF+4 and Dexron 4 are of a different viscosity and have different friction modifiers than the fluid specified by the manufacturer (Aisin-Warner). A few AW4s have been known to work on ATF+4 brand new, however there is plenty of evidence that ATF+4 causes slipping in transmissions that have been on the road for a while.
Dexron 4 may have some other differences... at the very least it doesn't match the same basic properties.
Quality of D/M fluid these days is pretty much what you'd expect from any other fluid. Cheap stuff will be cheap, expensive stuff will be high quality. There is a difference between the Royal Purple stuff and whatever WalMart got lowest bid for. If your AW4 is used for towing or the final drive is undergeared I wouldn't hesitate to seek out a synthetic to deal with higher temperatures.