Everyone has their logic. Here is mine. I based my selection off Mobil 1 Extended performance because it is readily available, reliable, and relatively cheap. Plus I believe that Mobil/Exxon has more money than any other company out there to develop lube.
If anyone has information about certifications and industry approvals for other brands, it would be nice to see for comparison's sake.
This PDF is gold
1. The Mobil 1 EP line only comes in 5w20, 5w30, and 10w30 flavors because these are the only viscosities that are stable/resist breakdown under a stable load. 0w does not last long enough to receive the proper industry approvals
2. I would consider these three viscosities to be within "spec."
3. protection: 10w30 > 5w30 > 5w20. Why? Look at the ACEA and industry certifications. Google them if you must. They relate to breakdown over time and under load. 10w30 is factory spec for supras.
4. gas mileage: obviously 5w20 will be better than the others due to a lower viscocity.
I would sacrifice mpg and run 5w30 in mine, 10w30 if reasonably boosted, and 5w20 at the bare minimum...