Originally Posted By: Buckeyeman
I did visit the Ranger forum, and got 10 different opinions. I've heard so many good things about the GM/Pennzoil fluid that I thought I'd try it, but I don't want to damage anything either. I once drained out the OEM fluid and put in Amsoil ATF and the dealer had to rebuild the transmission under warranty.
Synchromesh or a similar MTF definitely will not damage it. I heard stories years ago of M5ODs being damaged from being filled with gear oil. Early Rangers with Mitsubishi and Toyo Kogyo transmissions used gear oil, and those transmissions were used along with the M5OD from about '87-'91. Apparently some owners/shops would refill M5ODs with gear oil by mistake. All of that is anecdotal though.
The M5OD is not very picky and not very refined no matter what you put in it. Different fluids seem to work better or worse in different trucks used in different conditions. No harm in trying an MTF...if you don't like the way it shifts (doubtful), you can always change back to ATF. The M5OD came out in 1987, right around the same time Mercon was introduced, so Ford probably wanted to consolidate fluids on their assembly lines. Mercon could be used to fill a lot of things on a Ranger, Explorer, Bronco, Aerostar, E-Series van, or F-Series truck, including the M5OD. It was a good general purpose trans/PS/t-case fluid, but some M5ODs do better with something else. Mercon V was never really meant for the M5OD. It was introduced in 1996 or 1997 to correct issues with (I think) the 4R70W auto specifically, then was spec'd for other transmissions, including the ones used in Rangers.