going to change to syn rear end fluid what do you guys recommend and it does have an anti spin rear end do i need to ad an additve .

 

I've found on all my older trucks with a LSD (I have an Auburn Gear LSD in my current truck that specs conventional lube and the additive) that even if the synthetic stated you didn't need an additive that I got a little chatter around turns on occasion. After adding the additive it always went away, so I'd say to go ahead and put about 4 oz. of additive in anyway...

 

HammerZ71 is probably accurate however I switched to Royal Purple when I added the PML differential cover and although I purchased the Mopar additive I did not use it, and have not encountered any chatter. It may depend on which synthetic you choose?

 

Four quarts of Mobil 1 75w-140 (larger diff cover) and seven ounces of friction modifier. Easy project (easier than the tranny version).