Muncie 4 speed question

Quality brand 80-90 gear oil is all I ever ran in any of the M-21 trans I had including the one that I ordered new in my 69 Cutl*** S.

Putting a concoction in the trans for slicker shifting when racing or for extreme cold conditions may work but

Oldschool, there is a ton of info on Muncie M-21 / M22 Id on the net. A google search brings up more info than a guy wants to read.
here is one example which is pretty straightforward http://www.yearone.com/updatedsinglepages/Id_info/muncie.asp

 

Since hopped up used motor oil (10weight at that) instead of the recommended 90w, it would seem that there is only two possible outcomes. 1. Replace the 10 weight with the proper 90w and if that doesn't rectify the situation. 2. Replace the synchronizes, bearings, bushings, shims and the small parts kit.
Good luck

 

This is good for the older muncies http://www.summitracing.com/parts/RED-50305/

 

My opinion, use 80-90wt and check that clutch adjustment. Sounds like the clutch is hanging a little, not disengaging completely, Lippy