Is there really a difference between big-name gas companies' gas in the U.S., like Shell's and Chevron's, versus gas from companies like Valero, Arco, Rotten Robbie, etc.?

As someone who only used "cheap" gas for years, I feel like there is a huge difference when I switched over from Arco and Valero gas to gas from Chevron and, especially, Shell. However, I just saw a thread of comments where people were adamant that there is no difference in gas quality, and just the detergent blends are different among companies. Is my preference just a placebo effect?