Well I personally would not go 10,000 with this engine. The OLM should have been tested on the engine and it would have been done with conventional oil, and the EPA estimates done with conventional oil. If anything, people's MPG have done down and issues have gone up since putting synthetic in. If GM saw this much fuel in the oil I would imagine they would have done something about it. over 1% is absurd. I put convnetional oil in and will see if things improve. I plan to do a sample with 1500 miles on it and another at 5000 miles when I change the conventional. If that analysis comes back good I will keep conventional in for longer intervals.
If someone was insistant on using synthetic and staying with 5W-30 (since the manual says it has to be 5W-30) I would get the European spec 5W-30 Penzoil. Its pricey, but higher viscosity, hopefully keep fuel from getting past the rings, which is a completely common thing with high compression diesel engines. I combat blowby all the time with my diesel truck, and the more viscosity the oil maintains the better, it will seal the ring to the cylinder wall, for less blowby and less oil consumption.
I definately would not use a 0W oil, or lower than 30W when warm.