There aren't any standardized tests to document aftermarket product performance, so you do have to rely on the marketer's integrity. Some, like STP, have been visited by the FTC years ago and have since way back then been exceedingly strict and honest. Other, newer marketers are being visited by the FTC now; Slick 50 was only a little while ago, I think DuraLube is having a turn. Search for the Better Business Bureau National Advertising Division page; this is where many of these disputes start - companies challenging each other's claims.
IMO, if the FTC hasn't visited ProLong yet, they will soon.