I copied a link from a Studebaker club that was published by BW before GM started using over drives.
http://www.studebakerclubs.com/NorthGeorgia/Overdrive.pdf Page 19 addresses lubricant. It calls for 40w or 50W motor oil or straight mineral oil. I have run over the road trucks for over 45 years and the past 20 years I have used 50 W synthetic. This is what Fuller and Meritor requires to preserve the warranty. Before then I used 90 W mineral oil in transmissions. I use the 50 W synthetic in my Ford overdrives. When 90 W mineral gets hot, it is about the same viscosity as 50 W. Synthetic. I haven't seen 90 straight mineral around in quite a while.
