Wow! You've got great taste. I've got exactly the same truck, including color - with 4WD and the All Star Package.
After the free dealer changes with a new vehicle, I've always done my own changes. Right now, I've got an engine swap backed up, plus a lot of maintenance due on my other vehicles, so I've been less picky with the new truck. I'm embarassed to admit, I'm about to have the dealer do the third change using AC Delco Dexos (got a coupon for about the same price as the oil) at about 50% left on the OLM. I do have some experience with similar engines (the 6.0) in commercial applications, and have had to deal with an oil related problem, so I'm unwilling to go to low numbers on the OLM anymore, despite what others say. I freely admit, this is based upon superstition, not science, but I don't want to find out at 70k miles the OLM wasn't quite right for the AFM engine.
There is nothing wrong with M1, but the reason I'm convinced it's so popular is because of marketing. How many times have you heard "If it's good enough for a Corvette factory fill, it's good enough for me"? I won't use it for political reasons - they were in cahoots with GM in the blatent Dexos licencing (not certification) scam.
As many of the posters said above, any of the Dexos certified synthetic oils will do better than they need to in the 5.3. Once I've caught up on my other chores, I'll start using Pennzoil Platinum, which has a great track record (and I can get it on sale for a reasonable price).