The book calls for 80W-90 gearlube. If you are using synthetic lube, 75W-90 is the same thing with better low temperature performance.
Also, you want GL4 rated fluid, not GL5 rated fluid. GL5 is for rear differentials and can have additives that corrode brass.
GL5 tranny fluid did not corrode my Muncie synchros when I used to use it for many years. I also used it in the BW trans in the 77-81 Z28’s.
Best stuff on the market is Redline MTL (Manual Transmission Lube) specifically designed for earlier MT/'s https://www.redlineoil.com/mtl-75w80-gl-4-gear-oil
Amsoil has a lube very similar to the Redline product. Amsoil MTG is specially designed for older manual trannys and is GL4 rated.