I believe for your transmission, you want a mineral oil type gear oil like the one in the link sold at Tractor Supply.

https://www.tractorsupply.com/tsc/p...90-transmission-fluid-2-gal?cm_vc=-10005


Mike
1955 Chevy 6400 ex-flatbed (no bed now!) sold September 2023
In the Stovebolt Gallery
1958 Chevy 6400 flatbed W/dump
In the Stovebolt Gallery
1959 Chevy Suburban Owned for almost 20 years, Daily Driver -- sold May 2016
In the Stovebolt Gallery

Sufficient unto the day is the evil thereof