Old 50-60 year old Muncie trick, (works in T-10s too), use the intended gear oil with 10% STP. This would make the muncies shift like butter, as long as the blocker rings/synchros, were in good condition. I have no idea if that suggestion is still on the container of the STP, but back then, STP also recommended using 10% in manual transmissions.
Hot Rod magazine back in the day recommended the STP additive for fresh rebuilds. I have done it on my own cars back then, and it worked very nicely, YMMV.
Link to the same inquiry on another forum:
https://www.gtoforum.com/threads/whi...-speed.133083/