The best looking gears I've seen on the old stuff I have has been in the MTA's, 400's and 450's. The Hy-tran type stuff holds water but doesn't seem to react with it. The other decent looking ones (as far as corrosion) are those that have been sitting with the OLD oil in them that is just plain old oil.
TSC used to have some oil that was labeled for the old ford tractors, 8n 9n etc. I can't remember how it was worded exactly but it sounded like a GL-2 or 3.