I am an engineer in the refining industry. Here's the deal on gasoline. All gasoline purchased from a major supplier is created the same. Gasolines are blended to an ASTM standard and are subject to QAQC review by IDOT and EPA (for different reasons obviously). What MAY differ from brand to brand is the after-manufacture additive that is added to the fuel AND the cleanliness of the fuel. These additive "packages" usually contain some form of detergent, but in reality, are all roughly similar and equal in performance. Some of us believe that all the rhetoric about gasoline additives is more marketing hype than reality, but that is a subjective observation of mine.
To give you a feel for the impact of the additive package......the cost of adding an additive to gasoline is right at 2 to 5 cents per gallon, which means that very little additive goes into each gallon.
What complicates gasolines is that, according to regional environmental regulations, characteristics such as vapor pressure, octane, sulfur content, and other parameters ends up forcing the production of"designer cocktail" gasolines. If you live or purchase gasoline in an "ozone non-attainment area (i.e. large citiies), then most areas are required to add an oxygenate to their blend. The coastal areas of the country typically select MTBE as their oxygenate, while the "corn belt" states selected Ethanol (which is produced from corn).
There are two things outside the control of the gasoline manufacturer, that can result in problems. The first (and least likely) is the use of "old" gasoline. Gasoline reacts with air, forming compounds that are not burned cleanly in today's engines. So, you should buy gasoline from a dealer who does a high volume business. With high volume, he is constantly replenishing his stock with fresh product. "Old" gas used to be an issue when there were so many mom and pop gas stations all over America. Gasoline might remain in their tanks for months before being sold. Environmental regulations are cost prohibitive; which is why most of the smaller stations are being driven out of business. So, old gas from a station is becoming less of a problem. Another source of old gas is gasoine that you have stored for your lawn mower or other home engines.
The biggest culprit is, as one poster noted: WATER. All storage tanks will collect water, if the product does NOT contain Ethanol. If Ethanol is used as the regional oxygenate, free water in underground storage tanks is less of an issue, since Ethanol is hydroscopic.......meaning that Ethanol absorbs water that it comes in contact with. A real issue with Ethanol is that it will absorb only so much water. When the water content reaches the saturation point for the ethanonl/gasoline mixture, a phenominon called "phase separation" can occur and all the water will suddenly separate from the solution. When you hear the horror stories of a car filling up with fresh gas only to stall immediately.......the culprit is usually phase separation related.
If you purchase gasoline from a high volume dealer, the issues listed above should not be an issue.
Sorry for the long post.