Basically the ONLY difference between 5w-40 and 0w-40 is thethickness when cold.
0w-40 will be thinner when cold than 5w-40, but they will both be the same thickness at operating temp. Amsoil has a gurantee and no one esle can challenge that they are any better in quality or protection.
The first number is the cold temperature weight and the second number is the operating temperature weight.
5w-40 just means it is the thickness of a 5w oil when cold (thick as cold 5w oil) and it is as thick as hot 40w oil when hot (while 40w is a thicker grade than 5w it is still thinner at this point because its hotter, but it is thicker than 5w oil when hot) so it doesnt CHANGE viscosity as much as straight weight oil.
Mobil 1 0w-40 meets and beats all the specifications and test criteria for a 0 weight AND a 40 weight oil, without getting 'thicker or thinner'. Not bashing it at all, but it's not on the approved oil list because they don't submit to licensing for the standards. if you want more info, STFA BITOG and you'll find plenty of threads regarding amsoil's sketchy labeling and claims. Call Amsoil and VWOA and they will share with you and sort of info and documents that you require that it is approved. This has all been well documented and I agree it's very misleading.