The STP 80-90 weight that I was using is GL5 but is not synthetic.

Thank you. All the local big box parts stores by me only have synthetic

When I first installed this OD transmission it leaked real badly.  The same was true of the rear axle pinion seal which dripped continuously.  Both the transmission and the rear axle had sat uninstalled for many, many years.  Gradually, the leaks slowed down and then stopped completely.  Sounds crazy but the only explanation that I can think of is that the seals in the transmission and the rear end had dried up after having been drained and then left sitting for years.  Once refilled with oil, the seals expanded and plugged the leaks.  Anyway, I am a "happy camper" now as the OD is working and no oil leaks. Thanks to everyone who gave advise.

Here are a couple examples.  There are many more.  Check out manufacturer websites, look on the shelves at parts stores.

https://maximausa.com/series-283-premium-gear-oil.html
https://sharena21.springcm.com/Public/Document/18452/b05f7475-4d54-eb11-b800-48df3793b338/510adab1-8a54-eb11-b800-48df3793b338



Lawrenceville, GA

What GL5 isn't synthetic? I haven't found one..

THANKS!!

Wrong nomenclature. It's Ford Tractor Transmission Fluid:

https://www.tractorsupply.com/tsc/product/traveller-ford-tractor-hydraulic-oil-3-2-gallon

API Service GL-1. For use in final drives, hydraulic systems, and manual transmissions of Ford Tractors manufactured between 1939 and 1952

Dobie.....I checked the Tractor Supply Website but I don't see any transmission oil, only motor and hydraulic oil????

Gl-5 does not indicate synthetic and most modern GL-5 is safe with yellow metals.  If in doubt, refer to the manufacturer of the particular oil you want to use.


Lawrenceville, GA

Sometime in the past I read not to use synthetic oils, which GL5 are in OD trans. It was said they are too sticky and can effect the pawl engagement some how?. I've always used GL4 in my two OD trans.