I am running GC in my 06 Colorado 3.5. I just bought the truck two weeks ago with 41k miles on it, and it came from the dealer with PYB. So far, the engine is alot smoother at idle, and it revs like a raped ape.
I chose GC not for energy star fuel saving CAFE PC nonsense. I chose it for engine protection and longevity.
I have read that oils sold in the US for US market vehicles are engineered for fuel economy first, protection second. Euro spec oils are the opposite. Don't know if it is totally true or not, but it wouldn't surprise me. And seeing as I have no warranty anyway....