Putting 70/80 in a trans designed for 10/20 will definitely make it shift harder. After a while you will stretch the syncros and it may seem better, but a while later they won't work at all.

Because the syncros were probably poor to start with, you may be better off with a higher straight weight motor oil like 20w or 30w.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician