Come on now guys. Let me shine some light on this for you.
There is a pipeline which I am sure with all gas crisis here and katrina there you have heard of it. Well every company puts in X amount of 93 at the beginning of the pipe and then removes X amount of 93 at the end. They could be getting Chevron's or Amaco's or Shell's. Then they put in their own additive pack. THAT is the difference. The 93 AKI is 93 AKI at all the stations. They aren't cheating and putting in .5% extra ethanol just to lower the price. The discount places don't want water in their gas either, it is bad for business.
Some of you guys are saying "Online Top Tier for me" or "None of that cheap, discount gas for my car". Look at Top Tier website and see who is on their at the top. QuickTrip. One of if not the largest 'discount/cheap' retailers there are. They buy from whatever wholesaler is currently selling cheap and has fuel in stock and ready to load into a truck. So one week it may be 76, the next Chevron and the next Shell. They do have their own additive pack but to shun them is just dumb. I have a feeling that Sam's Club and Costco and Kroger all do the same. They may lack a little on the additive/cleaner/detergent side to save a couple cents but I don't see it being a problem.
I track my car, thus I am abusing it and revving it. Middle of July heat at Atlanta, on the track with a hot engine. I have used and NEVER had an issue with QuikTrip(my fuel of choice), Kangaroo, Chevron, Shell, Costco, Kroger, BP, Amaco.
So don't fret, your local discount station isn't going to ruin your car. Hell, the cap on the 96 328iC says run atleast 89... I put in 93 but seriously... To say you would rather run 87 of the 'good brand' over 93 from the 'cheap brand'. That is just being stupid.
Also on the running 93 and such... If it is tuned to run 91 then running 93 is just a little extra protection against pinging. You are NOT getting ANY sort of 'extra' performance. If you have a chip for 93, then run 93 or you may ping and have an issue. Running over the necessary is a great thing to do if your really beating on the car. 100 degrees outisde at Road Atlanta, my Dinan flash was 91 but running 93 leaves me a little safe spot in my mind just in case. Running it for performance reasons in a 328 or 325 that says 89 right on the gas cap? Doing nothing...