I run Redline with my 400# Eaton posi. The posi groans a little when you make tight coasting turns (such as into a parking space), but it doesn't chatter, so I don't worry about it. If I give it a little throttle to load it while turning the noise goes away. I had the same noise with Torco fluid with the GM additive. I've tried adding the GM additive to the Redline and it will still groan on tight coasting turns.
I prefer synthetic gear oil. I noticed a substantial drop in the temp of the rear cover (30F!) when I changed from the regular break-in gear oil to Redline synthetic.
The vendor who sold me my Eaton said the 400# units do make some noise in tight turns, it's just a side effect of the stiffer springs. A little noise from the posi in tight turns is OK, but chatter is not. That's hard on the clutches, spider gears, and your axles. If the noise suddenly went away on me I'd be worried the springs had failed or got weak.
I also run Redline with no additive in the rear end of my '89 Turbo Trans Am (Auburn cone posi) and it doesn't make a peep.
Redline, Mobil-1, AMSoil, and Torco are all good makers of synthetic rear end fluid that come to mind. Most say the additive isn't needed, but adding some certainly won't hurt things.
Mike, your comment of the rear end shop noticing unusual wear with posis and synthetics is intriguing (and worrying) at the same time. Can they give any examples characterizing the type of wear? I assume this wear is only on the posi unit's clutches/cones?
Old setup: Procharged/intercooled/EFI 353 SBC, TKO, ATS/SPC/Global West suspension, C6 brakes & hydroboost.
In progress: TT LS2, T56 Magnum, watts link, Wilwood Aero6/4 brakes, vaporworx, floater 9" rear, etc.