I had a 1989 F-150 with the MD05 tranny and it ran fine on the factory fill when I sold it with 116,000 miles on it. It shifted like butter. I replaced it with a 2002 F-150 with the same manual tranny, both trucks bought new. The 2002 shifts terrible with ATF. It's clunky and would grind in non winter temps(NJ). I finally replaced the second fill of ATF with RP Synchromax. This is about the same viscosity as ATF. The shifting is sooo much better, not perfect, still clunky at times but doesn't grind when it's hot outside. Most likely the tranny was not set up properly from the factory. ATF will not hurt this tranny but there are better fluids if you want to use them.
Whimsey
Whimsey