My thinking about changing the gear oil to synthetic is too gain some mileage. I get between 18 to just short of 20 mpg running 75 to 85 mph and in double overdrive at 1700 rpm... I know I could get better miles per gallon if I slowed down but... Thats just not fun...
So in reality, you are really looking for the beast fuel mileage, so then the best choice would be the 75w90 severe gear in that case. It's no slouch, that's what I run and I regularly gross over 18k in high heat
I've mentioned this before but Amsoil used to advertise a guaranteed 4.8% increase in fuel mileage with a gear oil switch, but with so many vehicles now coming out of the factory with synthetic gear lube in the diffs as a factory fill, they couldn't very well keep putting that on the bottle, kind of hard to have that kind of increase over another synthetic, but you will see that kind of increase over regular gear oil. The reduction in rolling resistance is quite a bit
