Gear Oil for 14 Bolt (9.5).

Just got a 3.73 6 lug 14 Bolt locking rear differential for 95 my K1500 as I blew up my 10 bolt 3.73 stock locking rear.  Before putting it in, I want to change out the oil.  Wanted to know whats recommended.  I was told that Shell Rotella 80w140 is what to use.  Was also told with a rear that old I should stay away from Synthetics. What do you think?  Keep in mind this is a completely stock unit with 100,000 miles on it.  Although I take the truck offroading, it is mostly street driven.

thanks

Tom  

Didn't know Rotella made gear lube.

Any brand name 80/140 will do the job. And there is nothing wrong with synthetics. Unless you have like a Detroit locker that specifically says to stay away from them then there is no reason to fear synthetic.

And I wouldn't run an 80/140 unless then manual specifically says to. That's a thick oil. I'd stick to a 75/90 or 75/110.  
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I think im gonna take back the heavier stuff and get the 75/90.  I originally figured to get the heavier stuff because this is a used rear with a lot of miles and figured a heavier oil would be better.