Do cars using Exxon gasoline get better gas mileage than cars using other gasoline?

Do cars using Exxon gasoline get better gas mileage than cars using other gasoline?
Gasoline(petrol) producers can use slightly different recipes for making gasoline, and those recipes change depending on the time of year. The main components of gasoline are hydrocarbons in their liquid state. This "straight" gas has an octane rating of about 40 which was acceptable back in 1930 but modern engines are usually designed to run on either unleaded(87 octane) or super-unleaded(93 octane) though some engines are "smart" enough to run both.
To achieve these octane ratings producers add a huge cocktail of compounds to the fuel for various different reasons, increase octane ratings, detergents for cleaning carbon deposits and such from the engine, stabilizers to allow the petrol a longer shelf life, oxygenates to ensure that the fuel burns cleanly minimizing carbon monoxide production and lastly among others, additives to increase fuel efficiency.
The production of gasoline is fairly heavily regulated with any new compounds to be put into petrol being analyzed by health and safety.
It is possible that Exxon might give better mileage than other brands but the difference would be minimal and i would recommend finding the cheapest pumps local to your area rather than brand loyalty