Is there a big difference in type F,A dextron and mercon fluids.will it hurt my 4r100 to run dextron.

 

This question comes up a lot on an old-car forum I hang out on, the real pros can chime in if I'm wrong on anything
All these types are almost identical in viscosity and in a pinch they'll substitute for each other, they have different additive packages that modify how they behave, and for best transmission life you need to run the closest you can get to the original spec.
Stay the hell away from anything labeled Type A, there's been no industry standard for the stuff for decades, anything you buy labeled Type A today is lowball 'economy' trash with a complete crapshoot when it comes to additives and how it performs
Type F is the Ford spec for older trannys and it's the closest thing on the market to the original Real Type A, Ford transitioned away from it in the early '80s, some draggers and hi-performance folks prefer it because it doesn't have as many friction-modifiers as the newer formulas so you get a quicker 'hookup', it might not be the best thing for a long service life in a newer tranny.
Dexron/Mercon is the formula that covers newer Ford and GM applications, it's got the modern package of friction-modifiers, anti foaming additives, and detergents and it's the spec that covers most Ford products from the transition years in the early '80s on up until recently.
If the bottle says it meets Mercon *too* then your Dexron fluid should be just fine in your tranny, IMO you'll probably never see any difference if you use fluid just marked Dexron III and higher.
There's a lot of 'discussion' going on about whether the new Mercon V spec will be okay in older trannys and how compatible it is with older spec fluid, IMO the jury is still out. You'll see fluid labeled 'ATF3+' or '4+' on the shelves, that is for Chrysler applications and has a synthetic component that may raise the same
issues as the new Mercon V......

 

Mercon V is the new standard and is compatible with the older trannies it is
basically synthetic and the additives etc. are better able to withstand the
thermal breakdown and oxidation. http://www.transmissionpartsusa.com/...-000062005.htm
That is basically the synthetic additives that make the V so much better and
can be added to convert m3 to V !!
If you are going to run a cheaper fluid to save money that would be a big mistake
since the price of a rebuild is too much to justify saving $10.00
I use a mity vac pump to pull a half quart to a full quart out through the dip stick
tube every motor oil change!! That way I have fresh fluid and additives flowing in
my golden transmission!! It only takes a extra 10 minutes and costs a couple bucks
and if it makes a trans rebuild even 5000 miles later than would be, it makes sense.