I have a 1956 F250 3/4 ton truck running gear (with a 58 cab and bed)
which weight of transmission and rear diff gear oil and which brand are you using?

 

Bookmark this page: http://www.clubfte.com/users/earl/Re..._Material.html
lots of good stuff on there.
There is a lube chart on there that says SAE 90 for normal temps. The key is it needs to be GL-4 spec, and as far as I know, only StaLube sells that, thru NAPA and some farm supply stores.

There are GL-5 85w140's available everywhere that claim to be "copper-safe", in particular Valvoline High Performance 80w90. Even Walmart's 85w140 claims to be safe. I believe them to be, but if you don't want to take a chance....

 

I got mine from Amazon; sta-lube gl-4 with free shipping

They have 140 and 85w90

 

Great advice! Thank you!
So GL-4 for both the diff and trans?

 

GL4 for the trans, it has soft metal brass syncros. I've used GL5 in the rear ends with no problem.

I also get GL4 from Amazon..