Costco is actually a major wholesaler and refiner of gas here in Northern California. Shell, Chevron and a lot of smaller gas companies purchase gas from the Costco refinery and add their own additives to it. You see their tankers pull up to fill up at their refinery.
So whether you want to use Costco or not, you are probably pumping Costco gas into your car.
I use Chevron or 76 but I have no problem with Costco gas. I just don't have Costco membership and the lines are super long for Costco gas. I try to avoid Shell if I can.