There's a few things you need to understand before digging too deep into this. Especially for those to whom this is all new info.
First, we're discussing the recommended oil for track days - not daily driving, weekend cruises...stick to Dexos1 5w-30 for that.
Second, Dexos1 and Dexos2 are GM approval certifications, nothing more. They aren't "grades" of oil.
Third, the performance engineers recommend 0w-40, or 5w-40 for their viscosity while on the track and operating at high rpms and high temperatures. But here's the catch: Dexos1 doesn't certify an oil above 30 weight.
Dexos1 is primarily for gas engines, and Dexos2 is primarily for diesel and Europe (emissions).
Normally, I'm a big fan of Mobil 1 0w-40 Euro blend...but I am planning to use the Valvoline 5w-40 specifically listed in the performance supplement, or the Motul 8100 X-Clean 5w-40 that meets Dexos 2 cert...unless something new comes out or earns a certification.
(I have no interest in running the Mobil 1 15w-50, and changing it every time I go to the track, though...)
EDIT: I find it very interesting that they recommend a Valvoline oil in print, because Valvoline fought very hard against Dexos1 certification when it first came out, even though they had oils that exceeded the requirements. That tells me the engineers did some homework with this one...