There's plenty of anecdotal evidence out there. Every can of snake oil has "testimonials" from someone.
Without quantifiable evidence, every claim has to be taken with a grain of salt.
I have, at various times, tried different additives in autos and kept objective records. None of them delivered any increase in fuel mileage or 1/4 mile ETs. I've taken engines apart at over 200k miles that have only had regular oil changes, and they are clean as can be.
The stories about STP gumming up the internals are myriad, and from what I've seen, quite true.
As far as fuel additives, the major brands, Chevron, 76, etc. have additives for cleaning fuel systems in them. They work as well as or better than anything you could add on your own.
