If your engine is anything like mine (3800 Series 2, NA) then that sounds pretty normal to me. I run full synthetic too at the viscosity recommended by GM (10w30) and I easily go through a quart to a quart and a half between 3,000 mile oil change intervals. Like you, I have zero oil leaking on the ground and I recently replaced both valve cover gaskets when I did the upgraded LIM gasket and UIM gaskets about 2 years ago. The engine is clean as a whistle and there's nothing on the ground where it's parked. I do know that if I REALLY get on it, it'll smoke a little bit from the tailpipe. I normally drive my car pretty much like a grandma, so that could just be carbon build-up. That said, I think GM might have designed it intentionally to have oil blow-by a little on the high side. Given how long these engines last when treated properly, it's probably OK. While extra oil blow-by isn't exactly environmentally friendly, it's definitely engine friendly IF that's what the engineers had in mind by design. Don't forget too that these 3800 engines are just like their smaller 3400 V6 cousins - GM specifically engineered them to where the piston rings will actually contract quite a bit to allow more oil to blow-by in the upper cylinder chambers when the engine is started cold. The obvious drawback to this design is increase oil consumption. My advice would be this: if she's running good and healthy and not giving you any problems or leaking any oil - just keep an eye on your oil level, be nice to the ol' girl, and keep on truckin'.