Rear end fluid?

[Hot Rod SS]

I'm getting ready to change the fluid in my rear end....... or I guess I should I say in my car's rear end. I'm using Mobil 1 Synthetic 75W-90. Anything special I need to know? The man at NAPA said I should use this small bottle of additive. He said it was for limited slip and positive traction rear ends. And would prevent the fluid from foaming. I've never seen anyone talk about it on here, so I wasn't sure. What do you guys think? Thanks.

[Fastcar]

GM has a synthetic rear differential fluid which my mechanic put into my diff. when I had the rear pinion seal replaced - it didn't need any additional additive. In this day and age I can't imagine that Mobil would formulate their synthetic fluid without considering the needs of limited slip rears. With a conventional dino fluid I would consider putting in the limited slip additive - my old AMX needed it but that was in the day when they didn't have Mobil 1 synthetic anything, much less gear oil. The additive helped prevent the clutches in the rear end from chattering when you went around corners. While you're at it, check your pinion seal for leaks; they all seem to seep a little but you should'nt have any major fluid drips nor should the differential housing have any major oil coating.

[2001NBMZ28]

Probably didn't need it, but put it in mine anyway - owners manual said to use it even with "GM Synthetic Spec........whatever." Won't hurt, kinda expensive for only 4 oz though.

[Hot Rod SS]

Thanks guys. Kinda what I was thinking. Maybe don't need it with Synthetic, but probably wouldn't hurt. And yes, $6 is expensive for 4oz.