Gear Oil Info

I’ve been running Amsoil 75w-90 in my entire drivetrain. 1956 CJ5 with OD, for 25 yrs without issue.
I recently read about the GL4 classification and synchros and got nervous because the Amsoil I’m using is GL5.
I found this test that made me feel better about GL5 in our rigs. Go to pages 16 and 17 for the results on copper with various brands. A few completely failed.
http://www.syntheticwarehouse.com/brochures/g2457_gearlube_study.pdf

 

I cannot say the truth but..
10 years ago I was working with Army people.
I asked to a guy working for an oil company providing NATO, he told me not to use synthetic in the old engines.
It was probably to protect some parts not used in new vehicles.
Some people know better than me. And it is probably only for the engine...

 

Yeah, there's no reason why the Amsoil GL-5 that's formulated to be compatible with yellow metals would be bad for the transmission, AFAIK. Your typical GL-5 dino oil is not compatible, again AFAIK. Some brands may be ok as shown in document above, but how will you know?

However, your T-90 will probably shift better with GL-4 dino oil. The trans and TC share oil, so same in both. GL5 is fine for the axles, dino or synthetic.

 

Perhaps but the ARB's use a copper tube air line connection. I'm using the same thing davet is in those.

 

A couple days ago I switched out the Amsoil to StaLube GL4 as Herm has this listed.
I’ve only put about 20 miles on so far.
I’m curious to see if it has any effect.
I don’t see in any of the Amsoil Severe Gear 75-90 literature mention of yellow gear compatiblity.

 

The Amsoil 75w-90 is MT-1 rated, which requires some amount of compatibility with yellow metals.
25 years with zero issues sounds like decent testimony that the oil is working. If the transmission shifts good, why change?

I don't know what the heck is in my transmission, but it grinds about 50% of the time going into 2nd and 3rd. I will be switching to StaLube after I rebuild my transfer case.