If it wasn't so expensive i would get some Redline MTL 75W-80, it even states on the website it is recommended as a replacement for SAE 80, with the Monograde 80 it's true that the shifting is a little notchy when cold, but perfect once the oil has warmed up a bit.
It would be a great choice, but since it's that expensive, making the change for a typical 75W-80/85 full synthetic will already be a nice jump compared to the SAE 80. You will be going from a VI of ~100 to ~160