When Castrol 0w-30 and Mobil1 0w-40 are used in the VQs, historically speaking they've provided very good UOAs. So you could go with either oil at the viscosity mentioned and should get very good UOAs.

I'm in the Northeast so I change my oil every 6 months or 7500, whichever comes first. Normally in November and May. I'm under warranty AND I don't want to change oil in the middle of the winter.

As I said, the best thing is to do an UOA. The $30 or so per oil change every 4K miles is cheap insurance, yes. However folks doing it when using synthetic, out of warranty, live in year round temperate climates like So Cal, Arizona, Texas, Georgie, Florida, etc who are DIYer's (like myself) without an UOA are just wasting money. Why not do an UOA so you can make an educated decision with your OCI AND you can also find out if you have any other engine problems, such as coolant in your oil, fuel in your oil, etc?