0NachoMahma 8 years ago ReplyUpvote. Many older transmissions (including Fords from the 1930s) contain what is called "white metal" which is not compatible with modern GL4/GL5 lubricants (although some say GL4 takes a very long time to do any harm). Use straight 140 weight if you can find it, something in the 75W-140 range if not. . I don't remember if the rearend has any white metal, but 85W-90 is about the right weight. . These recommendations are made without having a good idea of what you actually have (for one, I'm assuming a manual transmission; for another, no limited-slip differential). Find some old Ford datasheets for your units (if you are exceptionally lucky, the ID tags will still be on the trans and rearend, but don't hold your breath) and verify my info.