Been trying to read some up on the rear end lube for my dually. Thought I would replace it soon as I don't know if the PO ever swapped it out. What is suggested type oil and do I need some additive like I put in my posi rear in my Buick? Or no additive? What's a good type? I have 129,000 miles on it and would feel better changing it now.Could I go to a synthetic or is it a waste of money?