I have an 09 3.6L DI and there are a lot of posts here and on the Encalve, Traverse, and Acadia websites. As some have mentioned, you need an oil with as low of a NOACK rating as you can find. The 3.6L DI beats up both conventional and synthetic oils. I am one who feels following the OLM in a Enclave, Traverse, Acadia, or Outlook will result in trouble long before one would expect. You can search the used oil analysis to see how this engine beats up oil – not so much so in the Cadillac sedans but certainly in the Lambda platforms.
Thus, use a synthetic with as low of a NOACK rating as you can find (some have been mentioned in this thread already), change at 5-6k (assuming synthetic) and ignore the OLM. To best determine how long you can go considering your driving habits and etc. a used oil analysis is recommended. If you plan to have the vehicle no longer than the warranty period then screw it, use what the book recommends and change according to OLM.