The mineral oil sounds like a good way to go. Pennzoil still sells a GL-3 grade gear oil that would probably work well too. I just picked up a qt of 90 wt at the local Napa store for the M20 Muncie 4 speed in my C10.
1966 C-10 Short Stepside HiPo Step-by-step ~ and on-going resto build in the Project Journals
