I have always been a firm believer in synthetics but have heard from a few sources not to use it in a hi performance 12 bolt application.
More importantly I think is the GM additive, I once left it out of the rear on my '87 IROC after a fluid change and it was making some serious noises. I had to drain it and do it over again using the additive but that was just a stock rear.
Marc 2000 Camaro Z28
A4, Black, stock internals NA - firstname.lastname@example.org
(QTP headered, Cartek tuned) 2010 LSX Shootout True Street LSX Challenge 13.00 Winner 2006 Corvette
6 speed auto, Velocity Yellow - email@example.com Quickest bone stock Corvette LS2 ever?!
Now with some bolt ons and Cartek tuning - 10.7