I'm surprised your rear end guy didn't know about nor add the friction modifier from the start. GM knew about it and added it from the factory when these cars were built.
When I rebuilt the differential on my old Nova, I went with the Factory style Eaton limited slip carrier and then I spent an hour on the phone with the Owner/Chemist of Redline Oi, because I wanted to get it right. He assured me that his synthetic products were formulated such that the friction modifier wasn't needed. I went away fat, dumb and happy. That is, until my rear end started to chatter. I flushed and replaced the Redline with GM Posi-traction Gear Oil and their friction modifier. It all quieted down and all seemed well.
Two years after I sold the car, I found out that the new owner had to replace the Posi carrier, as the clutches were worn out and most likely this was due to the wrong product being used during break in.
Now, I'm not bashing Redline's products, but they are not the be all, end all. Whenever I'm in doubt, I find that I generally default to what GM knew and did from the factory. 99% of the time you'll do best by following their recommendations. After all, they had to warranty their products and that can cost $$$$.
When the only tool in your toolbox is a hammer, you tend to see every problem as a nail.