What motor oil do you use and why?

I use Shell synthetic in my Duramax because of the 7500-mile drain interval and how clean the oil remains over the time it's there. And, it's available at WalMart. I might use Mobil Delvac 1 synthetic, but it's much harder to find.

For a 6.5, I tend to use Chevron Delo because of the 3K drain intervals and the soot load.

Jim