Importance of Gear Oil Viscosity

[Overfill]

The SAE J306 can help highlight the definition for maximum cold weather viscosity. As I understand it, transmissions and differentials bear lubricants acting in the boundary lubrication regime. Assuming there are no pumps, clutch plates, or synchronizers for these systems, of what engineering significance is the hot temp viscosity?

 

[dnewton3]

Assuming there are no pumps, clutch plates, or synchronizers for these systems, of what engineering significance is the hot temp viscosity?

That is an excellent question, to which I don't have anything other than an anecdotal observation.

Performance Review Institute (PRI) - We Protect What Matters

www.p-r-i.org

(see the "LRI QPL" link therein; that will list all the currently approved lubes)
Also, there is this Program Document; a good overall description of the testing:

https://media.p-r-i.org/assets/2022/03/24100953/PD4000-REV-N-CURRENT-February-10-2022-v2.pdf

Per this standardization of this testing, all the grades of various lubes on this list pass the same tests. These are performance based tests, not content related. So the different grades all "pass" a very stringent threshold. That's not to say that the thicker grades could not somehow exceed the thinner ones, but there's no test to differentiate them in this standard, regarding grades.

Interestingly there are both conventional and synthetic lubes on there. Further, there are lubes which I suspect would do quite well, but the brand simply does not seek approval (HPL; Amsoil; etc).

I find this QPL list to be my "go to" resource. If it passes these tests, it's darn good enough for any "severe" service I'd put my gears through.