I put a can of the Motor Treatment
in the gas tank after filling up, and another can
down the oil filler neck (where you'd normally fill the engine with oil). After it had run for 3k miles, I replaced the oil. I didn't notice any difference in performance or noise, which I suppose is the best possible scenario
I think there's other ways of doing it, like letting a little get sucked up by a vacuum hose into the intake, but I haven't tried anything like that yet. I may if I end up feeling frisky sometime this summer
Let us know how it goes for you!