Getting back to the original question of which oil to use, I'd suggest researching Sequence IIIG engine testing or consult someone who is knowledgeable with this test.
The test looks at oil thickening (viscosity increase), piston deposits, oil consumption, and valve train wear. The test engine is a flat tappet 3.8L GM v6.
Test parameters are 3600 rpm with a 250 nm load applied for 100 hours. Oil temp is maintained at 150˚C, coolant at 115˚C, and afr is 15.0:1.
My point is, don't look to much at claims in advertisements, you may not need to spend the additional cash paying for them.
Off hand, I do not know the API rating of the reference oil for this test. I'll try to get this information today as that rating may give you a good indication of what a flat tappet motor needs with regard to additive package (ZDDP content).