25% difference in MPG as mentionned by the previous poster is highly/very/almost improbable, but probably impossible.

Remember that the gas you'll get at one station might not be the brand appearing on the street sign. There are thousands of exchange agreements between the gasoline retailer/refiners where, as an example, Shell (or any other refiner) would supply a certain region and in exchange, Cheron would supply in other regions. gasoline is gasoline is gasoline
All gasoline in a specific region must meet the local regulation and nobody is giving freebees. They meet minimum regulation.
High octane for a car that does not need it is a total waste of money. You'll get no benefit in term of MPG, power, or anything else.
Check this post for more info.
Premium gas?