I don't know man, my SL seems to run the best on Chevron Premium. I've also experinced something else that keeps me from cheap gas - a lot of those off-brand stations have been known to fill their high-test tanks with plus or regular. I've gotten a few pingy loads that way. The name brand company stations, big nasty planet-polluting oil companies that they are, seem to be some how totally honest in this regard and I always get what I pay for. You'd probably be ok with Costco, I do Ok with Sam's, at least in getting the correct octane. I've done enough computer work here in Houston to tell you this: neither one of them carry any particular "brand" of gas - they're grabbing whatever's low on the spot market, could be any brand.

Kirk
83 280 SL
84 Porsche 944
etc