Because of the reformulation all gas now has to have the same amounts and types of detergents in them. So if you buy gas at Arco (the cheapest in SoCal) it is essentially the same as Shell/Chevron/76/Texaco/Exxon. And the 87 gas has the same detergents in them as the 92 gas.

Reformulated gasoline (RFG) has nothing to do with detergents. It is gasoline that is reformulated by adding oxygenates such as methyl tertiary butyl-ether (MTBE) and other compounds that lets the gasoline burn cleaner. Here is a web site if you want more info: http://www.energy.ca.gov/forecasting/RFG/RFG_FAQ.html

The only thing that sets different companies gasolines apart is the additives they put in them at the terminals. Do the additives do anything? Well, they brand the gasoline so it can be identified who it belongs to in case of a spill or tank leak. Otherwise, nobody believes the additives do anything except the Marketing people.

George is right that all gasoline comes thru the same pipeline. The additives that are being added at the terminal are the detergents for the different brands.
These additives are important in keeping the injectors and fuel line clean. The question is then "Is there much of a difference in the detergent between Exxon, Texaco, and other brands?" I don't know the answer. There could be or it could be all marketing.

-PSU