I have pondered in the past about this very topic and more often than not, I can find "branded" oils at the same price or cheaper than the store brands (particularly with rebates and the like) so I have remained with a major brand. Now, do I think that store brands which meet API specification are lesser quality or we are somehow missing out? Nope, not in the slightest. Walmart Supertech is typically made by Warren or Citgo (in my area) and both of those companies make very good oil.
From a slightly different angle, I use Supertech DEF in my PowerStroke versus Blue DEF (Peak), Valvoline Premium Blue DEF, Shell Rotella DEF, or Motorcraft DEF. Do I think that SuperTech is of lesser quality? No, it is API certified and that is all that Ford states is needed to meet the warranty requirements. Why spend more if you do not have to?