When i dump my old oil at Checker auto, there is a sheet to list what brand of oil you are dumping. On a full sheet, there were about 15 Valvoline, 3 Mobil (not Mobil1?) 2 Havoline and 1 Castrol. I guess "more mechanics use Valvoline." Why no Pennzoil if it is the number one seller? Does Valvoline sell that much compared to the others?