Gears & Axles - Burned up a ring and pinion at the track

Went to the track not too long ago and as soon as I hit the road to return home, I noticed that my R&P was whining severely. The gears had been rather quiet up until then BTW.

I only made 5 passes roughly 20 minutes apart with #2 & #3 being back to back. I was running plenty of 75-90 Royal Purple syn oil in my 9" rear end. The gears also had a couple thousand miles on them.

Ended up pulling the center section out and found that the gears were totally destroyed and the housing was filled with metallic sludge. I also determined that bearing preloads were still set and nothing had moved internally which would have caused this problem. Backalsh went from .012" to .035" as a result of so much tooth surface being worn away.

Could making back to back runs cause them to burn up like this?
Should I use standard oil or synthetic?
Why did this happen?

I have 12 bolt and I was told synthetic is no good and to use 80 w 90 standard oil

running a synthetic in those built rears like a 75w90 will be like running water in it. It's actually recommended to stick to a regular dino oil.

Nuff said.
Will stay with the 80/90 dino oil.