Pinion Seal Replacement

Finally got my carrier gasket, so hopefully be able to get some wrench time on the car this weekend.Final question is about rear end fluid.I'm picking up a bottle of the Mopar Limited Slip additive for the SureGrip.As far as gear oil, I was planning on using Castrol Hypoy C 80W-90. Now, it says on the back it meets requirements for conventional AND limited slip diffs. Exceeds GL-5 & MT-1 specs.Has anyone ever used it? Since it says it meets requirements for limited slip do I still need the additive? What could be the drawback of using the oil AND the additive if the oil already has some sort of additive in it?Thanks