I once used RP gear oil in my 5-spd manual. I was blissfully ignorant that what I had was not Synchromax. It was terrible in the cold. It shifts hard. On really cold mornings you hear whining from the transmission. You can also hear things clicking into place when you shift. Don't do this... (Duh)
This summer I added 50% Redline MTL and 50% Redline ATF. This combo has provided good results as per a number of posts on this forum. Mine shifts very smoothly and quietly now. I plan on changing the tranny fluid every 1-2 years.
I love the smell of brake cleaner in the morning...