If you are not familiar with Top Tier gasolines you should be. I won't go into all of it here but here is a link to get you started.
Top Tier Gasoline
It is well worth reading and noting which gas companies in your area are Top Tier gasoline companies. To be listed as a Top Tier gasoline company all grades of gas you sell must meet the standard. A cleaner engine means better gas mileage. I use to keep a list in my car of the companies but now I know what is available where I drive. I usually just stick with Shell which is also sold at some Circle K stores here in Arizona.
One more link that is a little more up to date about which gasoline companies are Top Tie and a little more info.
Drive it Forever: Top Tier Gasoline | Wise Finish
Al