I was traveling with a co-worker a few weeks ago, and he said he likes to fill up his gas tank at Exxon gas stations because it seems to last longer.  In my 26.33 years on this planet, I don't think I've ever heard a stupider thing babbled from the mouth of a fully grown, college-educated bi-ped. 

However, after doing a very small amount of research, I've determined that it could possibly be argued that different brands of gasoline yield different values of gas mileage.  It's a widely known fact (though not known by me until 7 minutes ago) that oil companies each add their own special concoction to the gasoline they sell to produce a varying array of cleaning functionality and gas mileage improvements.  One can conclude that with the proper amount of controlled testing, a definitive set of results can be obtained to link a certain oil company's gasoline with increased gas mileage in a certain vehicle under certain driving conditions. 

However, it's safe to assume no amount of controlled testing was done in my co-worker's case, which means he probably got 9 mpg in his stupid pickup truck (misplaced anger, anyone?) when he filled up at the Citgo and drove to and from work for a week, but got 11 mpg when he filled up at the Exxon and drove on the highway for 300 miles.  Not to mention the fact that there's no way car companies would design engines to effectively utilize one company's gasoline additives over another, which means that different engines will work better with different additives, which means independent studies would need to be conducted that measured the effects of each brand of gasoline in conjunction with each brand of engine (and each type of driving, i.e. stop and go, fast and steady, etc.), which just further cements my existing practice of just buying whatever's cheapest and closest.