[Msquared]

The x3 and x4 versions of the 0w40 may have the same fundamental ratings as the Supercar 0w40, but they are not the same. The x oils have better high-temp/high-shear viscosities, and testing has confirmed that they resist viscosity breakdown better than the Supercar version. It probably doesn't matter at all for basic street driving, but if you track or even autocross your car, the x4 is the better option. It's also a great deal at Autozone (they always had a deal on x3 in the past, too). The lower calcium (a detergent) improves resistance to knock, as well. I'm really not sure how the new European 0w40 compares.

Since I relegated by 1LE to weekend fun (no more daily driving) and increased my track days (as well as ~15-20 autocross per year), I have started using the 5w50 Supercar. It has significantly more zinc, better HTHS, and higher 100C viscosity than any of the 0w40 options. However, as Rich292 pointed out, it's 40C (104F) viscosity is quite a bit higher than any of the 0w40s (112 vs 72). If I were going to start daily-driving again, I would probably revert back to x3/x4.