This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
The Toyota FAQ (Todd Haverstock) sez: "Independent labs as well
as engine manufacturer Briggs and Stratton have rendered a
verdict that Slick 50 and similar oil treatments are useless."
The Gasoline FAQ says most gasoline additives are useless.
Others have weighed in on that score, and DuPont sued to prevent
Slick 50 from using Teflon (unsuccessfully). For more details: