Reference: Manual Transmission Oil

[Jacques Reed]

As I am about to swap the gears from one transmission case to another on my Ford F15-A I thought it would be a good time to revisit the type of gear oil to use.
After reading this thread I started hunting around on the net for more info. A lot of info that I read about EP additives to gear oil dealt with damage to bronze synchronizers in synchromesh transmissions or bronze worm gears.

In a Ford CMP truck with a non-synchro transmission my main concern however is possible deterioration caused by those additives to the reverse slider gear bronze bushing and the bronze thrust washers in the end gears in the differentials

Extreme pressure rating GL-4 gear oils have about half the quantity of buffered sulphur phosphate EP additives as GL-5 rated oil so is less harmful to yellow metal. As the rating indicates however, GL-4 Extreme pressure protection of gears is less than GL-5 rating.

The next question: Is GL-4 rated 80W-90 gear oil equivalent to, or even better, than the wartime SAE 90 gear oil as far as gear tooth protection goes? If so only GL-4 rated gear oil seems the obvious choice.

Ford gives a spec in their Special Pattern Vehicles manual as Ford spec MC-568A and call it “Extreme Pressure Gear Oil”. Haven’t been able to find a copy of it. It would probably be difficult to relate it to modern extreme pressure ratings anyway.

I have been using Castrol EPX 80W-90 in my CMP truck but it is GL-5 rated so it would have double the yellow metal damaging EP additive compared to a GL-4 gear oil. I found 3 suppliers of only GL-4 rated gear oils in Australia. Penrite, Hi-Tec, and Shell make them and market them here.

On the positive side, the reverse slider gear bushing seems fine even after being immersed in GL-5 oil for 10 years but then again I cannot see micron size damage! Hopefully ditto for the end gear thrust washers.

Any ideas on the subject gratefully received.

Have attached the link to the paper written by Richard Widman a Chevy Corvair enthusiast and a link to a Machinery Lubrication article on EP additives. Both are very informative on this subject.
https://www.widman.biz/uploads/Transaxle_oil.pdf
https://www.machinerylubrication.com...itives-effects