I drove an '08 3500HD wrecker at one place I worked at. It had the 6L gas engine in it (therefore it didn't have Allison trans, not sure what it had, but it had five gears I think), and it too seemed to have a transmission issue intermittently. It would "flare" or slip slightly when up shifting from 2 to 3.

It was a good looking truck, and I loved driving it, but at 8mpg (!!!) I'm glad it wasn't mine. The Freightliner flat bed I also drove got 9-10 mpg, and was easily twice as heavy!

It went like a scalded cat, high on crystal meth though, and I could almost beat the police to a call if I wanted!

It had a weird banging noise (like you were beating on a steel garbage can with a baseball bat) occasionally, going slowly around moderate turns, and would violently shake the whole truck. It had something to do with the limited slip diff not unlocking. Of course the dealer didn't want to do anything about it, and basically said that it was impossible, and we were making stuff up.

I hope the dealer is going to help you with your excessive oil consumption issue, as that seems very high. If it was still in the break-in period, that would be normal. This sounds stupid, but try driving it hard for a couple hundred miles. Piston rings might be stuck or something. I would also get a compression check done on it somewhere other than the dealer. It is sad that GM dealers don't care about their customers as soon as they drive off the lot.

I personally like GM vehicles, but the way they treat their customers is DISGUSTING! And I would never buy one new.