I had a 1989 F150 with the MOD5 tranny. I shifted fine with the factory Mercon. My 2002 F150 with the same tranny shifted horribly with Mercon. It was better with Schaeffer's 204 ATF, but not good. With both the shifting was ok in freezing temps until the fluid heated up. When warm it crunched & grinded, more so with the Mercon. I ended up trying RP Synchomax MT fluid. It's way better, not perfect but way better. From what I've read here and other sources a correct viscosity dedicated MT fluid is better for the manual tranny than ATF. It has the additives needed that ATF does not have for a manual tranny.
Whimsey