CL,
Oil companies make their lubes in the standard SAE grades recommended by engine manufacurers. For the past 10-15 years, the vast majority of US and Japanese manufacturers have recommended 5w-30 and 10w-30 grades for gas engines and 15w-40 grades for HD diesel engines. European engine manufacturers have continued to recommend a variety of SAE grades, depending on the ambient temp range the vehicle is being used in.
The M1, 0w-40 is made for those European vehicles that recommend an oil meeting the ACEA "A3/B4" and VW 502/505 specifications. If you are experiencing oil consumption or oil pressure problems running the M1 5w-30/10w-30, then the 0w-40 would probably help to some degree. It runs about 20% thicker in terms of high temp, high shear viscosity - this reflects the thickness of the oil in the bearings.
The additive chemistry of these three formulations is virtually identical, so I would not say the 0w-40 is necessarily a better product. All things being equal, the 10w-30 is going to be the most thermally stable and it's properties should change the least with use ....