When I was a kid, I heard the old timers talk about adding mothballs to the fuel tank of the Model T to increase the octane. One could then set the manual spark advance control (the spark advance was on the steering column) and the Model T would really fly–maybe as fast as 50 mph.
I doubt that the mothballs really did much good back then and I am certain that with today’s fuels, the additives do even less good today.