Well, how does the viscosity of the 85-140 at 50 C compare to the viscosity of 75-90 at 40C. Or film strength, or whatever lube property governs? If we take the spirax graph at face value, the 75-90 was still climbing.
I guess it's relevant because an axle or transfer case (unlike a trans or engine) is not thermostatically controlled. So if the fluid makes it run hotter, you'll get the fluid properties of the hotter unit.
I will never believe that the dominant source of heat generation inside of an axle is fluid shear, as opposed to bearings rolling/sliding and gear meshes sliding. (Even less so will I ever believe it inside of a transmission or engine with obvious other sources of heat generation/dissipation.)
I do not care if my rear end runs 10 C hotter. I care that the moving parts are well lubricated as it supports tools down the road supporting 6000 pounds, at GCW of 19,500 pounds, safely hauling my family on vacations down the highway. The temperature will be a function of my right foot, terrain, ambient temperature, airflow, etc. I do carry an IR gun in the console and when curious make sure all my trailer brakes are contributing (and not dragging) and when super curious, shoot the back axle.
In my case, an AAM 11.5" has had a variety of lubes specified over its history and depending on the OE using it, TSBs applicable, etc.
