Using the wrong fluid in any trans is a realy bad idea. I have seen people ruin transmissions rapidly doing that. Transmissions and fluids are designed to work as a matched set or at least that is how people should be thinking of it. And yes, many trans shops and parts houses can tell when the wrong fluid has been run. and often even if the parts are washed because of the way things wear.
I use a good brand of whatever the transmission calls for. Always Dexron III for GM automatic units like TH125c and 4Txx (4TxxE). Even many Fords don't use F fluid any more so watch out any time you work on one.
If you are beating the thing the odds are good you will break something simply from the beating no matter what oil you run. Automatics aren't made for abuse. If you are going to abuse them expect to pay allot of money to have them profesionally built up and even then expect they will break.
You might get a little more playtime from Mobil 1 or something else but I wouldn't count on that. Dexron III is already rated 30,000 miles at least as long as you don't cook it. Cooking any trans fluid will shorten the life of the fluid and the transmission rapidly. (There is a table linked to the trans or the cooling system stuff in my cave. don't remember which right now. maybe both...)
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