I agree 100% with George on this issue ....
I consider the Amsoil 0w-30/5w-30/10w-30 synthetics to be interchangable. The high temp properties of the Amsoil 5w-30 and 10w-30 are so close that the main reason I see for offering both is simply customer demand (I suspect this is also the reason why mobil markets all three SAE grades). I basically switched all my Amsoil 10w-30 customers over their Series 2000, 0w-30, as it does provide a fuel savings benefit.
The 0w-30 and 5w-30 grades primarily achieve better fuel efficiency because they flow more easily while the engine is warming up. They also flow more easily any time your oil temps are below 100C - the standard test temp for engine oils. It can take 10 miles for the oil to come to an equilibrium temp - especially in cold weather.
I have studed the Maxima data fairly closely and it is difficult to draw many conclusions, since there are so many uncontrolled variables (driving conditions, mechanical condition of different engines, miles on each sample, etc). The wear rates for the Mobil 1, 5w-30 and 10w-30 look pretty much the same if you go by that data, which is the most important thing. One of the more interesting things about the baseline data is that several of the API licensed synthetics, including Castrol Syntec and Valvoline Synpower, don't meet the 1000 ppm limit for phosphorus!