how hot should my rear be?

Stormin' ..I am right off the NYS thruway in Farmngton.
I spoke with the guy who built my motor (who is a great mechanic) and the guy that built the rear. Both guys agreed that it sounded hotter than it should but nobody could really say for sure how hot is to hot. The guy that rebuilt the rear suggested that I change out the 80-90w NAPA gear oil that I put in for synthetic 80-140. He said he does alot with race cars and they now for a fact that the synthetic reduces the heat. (I am not worried about the heat so much as what's causing the heat) He also said that maybe it needs to break in a little....there is less than 200 miles on the rear. I will also check my backlash.