The odd thing is that 75W-140 syn has replaced 80W-90 as factory fill in Sterling (Ford) axles. I think this was done in order to extend the change interval and protect better in heavy towing situations.

SAE 140 would increase drag at operating temperature, negating the advantage of the thinner 75W when cold. RedLine had a chart showing the mileage increase can be significant for short trips in cold weather.