Walmart's oil price is set not by the "quality" as perceived by BITOG'ers, it is set by the profit margin. If the distributors/producers raise the price of certain brands Walmart will raise price for those brands and when it didn't sell, they will do clearance and stop carrying those brands.
We all know that M1 0W40 is higher quality than 10W30(different application of course), Walmart sells both at the same price because their cost is the same for both.