I had a 1989 F-150 with that transmission that shifted just fine with the Mercon fluid. I bought a new 2002 F-150 with the "same" M5OD trans and it shifted horrible with the Mercon right from the factory. I finally ended up using Royal Purple Synchromax MTF fluid and it helped somewhat. I think the important thing is to use a MTF instead of an ATF. The MTF has appropriate additives for manual transmissions. MoleKule has a listing of MTF's and their viscosity so you can try to match one with a viscosity similar the the Mercon ATF.
ATF for freshly rebuilt M5OD
- Thread starter Creekside
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From what I've been told, the problem with gear oil in transmissions like the M5OD is that they have caged needle bearings which need a lighter viscosity oil.
The Royal Purple Synchromax MTF has the same viscosity as the ATF fluid Ford recommends, but better gear protection for a manual transmission.
I ended up getting Delvac 1 ATF for it. My local AAP had to bring it in from another store, which was not a big deal.
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