I recently read a pretty thorough study on the Internet that debunked all
additives like Slick 50. Sorry, but I don't have the URL, but you can find
a thread in the rec.autos.tech newsgroup in which one of the participants
gives the URL (and before creating a thread here, I suggest that anyone
interested go to that newsgroup first).
According to the study, as I remember, there is actually some danger of
these additives being detrimental to your engine, as the PTFE (Teflon)
particles they contain can clog your oil galleries, fuel filter, etc.
Although there is some evidence of improved power and gas mileage, these
gains are offset by the increased heat and wear caused by decreased oil
flow due to clogging. DuPont has tried to get these companies to cease and
desist using Teflon for these purposes, but the courts have ruled that
DuPont cannot restrict the use of its product in that way (restraint of
trade). The hawkers of these goods claim that the court ruling legitimized
their product claims, which it didn't.
I am just repeating what I have read, so the usual disclaimers apply.
However, I've got an old bottle of this stuff sitting on a shelf that I
would be glad to sell you for half the current market price;-)