Reference you inability to find 93 octane anywhere but 7-11. You may want to read the article on Top Tier gas. I use Shell V-Power, a Tier 1 gas, and I have noticed a significant difference in performance and gas mileage versus Exxon, and FINA. Chevron also is a Tier 1 gas. All manufacturers of Tier 1 fuel are listed in the article. Hope this helps.

http://www.toptiergas.com/