Sorry guys, I hate bringing up dead threads but there aren't a ton of posts on this forum, so...
http://www.toptiergas.com/retailers.html
List of "Top Tier" gasoline providers. If it's not on the list, don't put it in your tank. Costco is on the list. More than likely Costco buys whatever surplus supply another Top Tier provider has, and then uses that at their stations. In other words, Costco gas will probably be a blend of other Top Tier providers, so nothing wrong with that.
Just noticed that ARCO is on the Top Tier list as well. Damn, the sky is falling...