Jason,
If you have an application that actually requires a shear stable, mid range 40wt oil, I'd use Delvac 1, 5w-40 over the M1, 0w-40. Delvac 1 will stay in the 14-16 Cst range for a long, long time. The M1, 0w-40 will definitely shear down by 10%-15%, particularly in a turbocharged gas engine. In less severe applications, it maintains its viscosity pretty well, however for those the M1, 5w-30 or 10w-30 will also work fine. I would certainly not use the 0w-40 in a wet clutch motorcycle application that called for a 10w-40 synthetic ....
TooSlick