Most synthetic gear lubes will not require additional additive. I know that the Amsoil Severe Gear has it in it and lube like Royal Purple are supposed to also. It will not hurt anything if you add it and you do not need it.

 

talked to a service tech and he said you have to use limited slip oil or atleast that is what gm puts in it

 

Dont know about the truck, havent had to change it yet. But I do know that in my Chevelle (12 bolt posi) using synthetic AND the friction modifier did not work out well. Regular required one if not two to prevent clutch issues but using the additive and the synthetic just flat out made it too slippery.