85w140? As long as the climate tolerates it, it'll work. The questions I have are: diff requirements, diff family, diff intended usage, maintenance intervals...???
And, I don't believe "Seems that some gear manufacturers and installers are saying to stay away from synthetic gear oils. According to them the synthetic oils don't transfer heat from the gears like dino does." [censored] and foolish to believe it!
Guess which oils are the best heat transfer fluids?
Usually what manufacturers only think about is COST! And, installers are pretty clueless and only believe in hearsay.
I do think that some synthetics flow 'too well'. And, they might lack some cohesive/adhesive properties needed when there is a leak and/or lack of maintenance causing the level to keep dropping lower and dropping lower... That would be called failure by 'owner' and not failure by 'fluid' type. Blame a blender if they didn't consider that co/adhesive requirement.
Concerning heat? Viscosity, sump level, cooling limitations, and additive package, play a more important role than base oil used.
Monitoring gear oil temp will help you avoid 'fluid' failure but doesn't tell you the temp of the gears themselves and when they're going to fail. But, gears will fail when the fluid fails as there will be wear&temp runaway. You are less likely going to have a temp related fluid failure with better base oils and usually with thicker gear oils.
I'll stick with full synthetics and go thicker as/if needed. And, if I plan on running the fluid a long time(which is usually by design for a diff), I'll make an effort to check the level every now and then, and top off as needed.
