When I was a Honda tech back in the mid 80's they ran straight 30 weight in all their manual boxes and they shifted like butter and lasted half a million miles Never tried it in my boxes but I do run one quart of 30 weight motor oil mixed in with the mineral based 75-90. I've see a few cluster gears flaked where the rollers ride from ATF.
I ran the Redline in my Richmond when it still had syncros and I didn't like the way it power shifted.
Gus