Be as cynical as you wish. Your perogative.
I would agree that if I heard of a car that had been getting 20 mpg over the years and then a "magic" additive increased the gas mileage 20%, I, too, would be skeptical.
On the other hand, if I heard of a car that was designed to & had in the past routinely gotten 26 mpg and had a dirty sludged-up engine and was getting 20, and then a cleaning treatment loosened up stuck rings, improved the performance & compression, and the mileage quickly RETURNED to 24, I wouldn't be shocked or dismayed.
I'd be impressed.
I am impressed because I've seen it work.
Life (as we know it) is often a bell-shaped curve: as in all things, your "results may vary"...
Nothing wrong with being cynical: nothing wrong with the testimonial as a form of recommendation.
'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435