I don't use (or need) any of the oils listed, so although it may sound like I'm being partial I really am not, I'm just tossing another option or perspective out there.
I think it really comes down to what was the ultimate purpose of the test, does Amsoil want to show that they're the best diesel engine oil period. Or do they simply want to show people who buy their own oil that it's worth it to step up to Amsoil?
Redline, Schaeffer's, and others make excellent quality oils. But my initial impression is that Amsoil is not trying to sway customers away from them, but rather convince customers from the auto parts store oil isle to move up to their level.
So I don't believe Amsoil is trying to steal customers away from other "boutique" oil companies, but rather trying to convince customers to move up to the "boutique" level.
Again, don't flame me, I'm just saying there are other possibilities.