You think they're adding Hydrogen Sulfide? ;-)
Interesting story, probably unrelated. When I moved recently I had an extra half gallon of Methanol (Methyl Alcohol) that they use in some racing cars. Not wanting to transport this highly volatile and flammable juice, I put it into my nearly full gas tank. My commuting gas mileage went from 27-28 to 32-33 and has stayed that way ever since. I'm guessing it cleaned out some gunk from the fuel system lines or something.