What kind of gear oil for 8 3/4? Why not synthetic gear oil?

I have a recently rebuilt 8 3/4 rear with new Richmond 3.91 gears and a Detroit Truetrac (no clutches or cones...all gear driven) limited slip differential done by http://coastdriveline.com/ These guys are awesome BTW and they suggested not to use any systhetic gear oil at ALL. I totally respect their advice but I had already put Redline 75W90 synthetic gear oil in the differential before I was told months later that synthetic is bad for my Mopar 8 3/4. Luckily the car has not been driven yet. ](*,)

Redline says its 75W90 Gear Oil contains a friction modifier which is recommended for limited-slip units.

Why is it recommended you not use synthetic gear oil in these old mopars with limited slip differential?

Just wondering about this before I drain the Redline stuff and replace it with non-synthetic gear oil.

Dave