I'm not a 4 speed guru by any means, but if I were having that problem, the first thing I'd do is check the air gap in the clutch.

If that checked out good, I'd dump the trans fluid and try some Pennzoil synchro-mesh gear oil.

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads