I would honor the 6,000 mile factory interval for two reasons.
First, that's what they say you need to do to maintain your warranty.
Second, the recommended oil is for a GF-4 or better. GF-4 and GF-5 oils (Mobil 1 is GF-5) are full synthetics. Forget the extended oil change claims by oil companies because they aren't including turbocharged engines in that extended claim. Turbos are red-hot at the turbo shaft and only full synthetic oils can withstand that much direct heat and not congeal on the turbo shaft and bearing which is the main cause of turbo failure. The turbos are included in your 10 year / 100,000 power train warranty so I'd respect the 6,000 mile factory mandate.
I plan on keeping my GT2 for a long time, so I'm shooting for 4,000 mile intervals which I rarely make, so going over a bit won't be a big deal.