It all has to do with overhead.
Why is the storebrand less expensive than the namebrand? Because of overhead = packaging, advertising, etc. True, there are some namebrands that are a better quality, so even though you are paying more it is a better value. However, the only way to find out it to give them both a try. Will your friends KNOW that you are putting "Brand X" in your gas tank? Well, if you have a knocking sound, run-on and a bad odor emitting from your car then yes, they will know. But believe it or not I have had this experience from SHELL. Not all Shell stations, only one of them. Obviously I stopped going there.
All the gas stations are privately owned so if the owner is going to rip you off by "fixing" the pump or watering down the gas you won't know it until you try it. And even when you do, what consequences will he suffer? Are you going to call 20/20 and have them investigate the station on hidden camera? If the owner is unscrupulous his profit margin will not decrease as most people will never catch on to what he is doing so there are no rammifications to his behavior.