After reading the article in the last issue of Vintage Ford regarding the possibility of GL-5 gear oils damaging yellow metal components in my differential, I figured I had better get a hold of the Amsoil Tech Department and get their twist on it. I've used Amsoil 85W-140 in my Ruckstell since I rebuilt it with a Chaffin's kit. No problems, no leaks. but that didn't mean something bad could be lurking ready to bite me in the rear end (so to speak). So I emailed the following to Amsoil and here is their response:
The article has some truth in it, but there is quite a bit i=of information missing. In order for the EP Gear Lube to affect yellow metals such as bronze, the lube temperature must exceed about 275 Deg. F. Manufactures must weigh the need for the EP additives against the possibility of high temperatures and yellow metal corrosion against each other. In the case of your Ford Model T, the likelihood of high lube temperatures is non existent, so AMSOIL 85W140 GL-5 gear lube can be safely used.
Back in Model T days , thick 600W non EP gear lube was used as EP gear lubes were not yet “invented” and they wanted the thicker oil to slow leaks and provide more wear protection. You will have no bad issues by using the 85W140 gear lube.
Thank you for contacting us. If you have further comments or concerns, please contact us again.
ADDRESS: 1 AMSOIL Center, Superior, WI 54880
Well, I never gave it too much worry myself, but it's good to hear it from someone who ought to know, rather than just going by my gut instincts.
Thanks for asking an expert
I've never seen the temperature aspect mentioned before when the GL5/brass problem has been discussed multiple times online.
Sta-Lube makes a 85w-140 GL4 gear lube that will work also.
I have used GL5 lube in my '15 with a Ruckstell for the past 20 years, and it probably had it prior to that. I doubt the ability of it to damage anything.
There was discussion on that subject here about 15 years ago and it came out that GL5 may make the outside surface of some yellow metal parts turn gray after many many years.
But it will not destroy the yellow metal parts as many believe will happen.
I mentioned GL4 transmission grease to my local, old fashioned, service station guy.
He said he remembered using only GL5 in his over 40 years in the service business.
That was good enough for me but, I too, had never seen the temperature factor explanation as to why GL5 might affect brass.
Something to read for you.