My 85 Iroc has the factory 10 bolt with 3.73 posi. Here lately its started chattering some and not always locking both wheels. It used to lock up really well. Its got 145,000 miles on it and I know the best thing to do is to R&R with an Auburn, etc but that's out of the question for now. My question is I've seen somewhere where someone was having about the same problem and they drained the rear end, 'cleaned' the clutches and everything, put in new lube and it made a world of difference. Has anyone done this and what was used to 'clean' everything? I'm needing to replace the pinion seal and this would be the time to do it. I thought about spraying everything with brake cleaner, blowing it dry with an air hose and then refilling. Also, anyone use Castrol rear end lube? It claims its for limited slip applications and doesn't need the additive.

 

I think you'll find that the Castrol lube (or Valvoline, or Coastal, or about any other) says on the label that it's "compatible with" limited slip rears or some such; but none of those have the additive. All of them require the addition of the friction modifier, without which the clutches will chatter and groan and otherwise misbehave.

Definitely change the fluid; add a bottle of GM friction modifier, and then fill it the rest of the way with good quality fluid. I highly recommend using synthetic instead of dinosaur juice. It is said that Amsoil does not require the additive, but I have no first-hand proof of this; all others will definitely need it.

I usually use Mobil 1 or Red Line.