For a clunker on it's last legs, sure, why not?
Any other car, I agree with Jack and csnow, "no".
This topic can almost get as heated as the "is there a god debate" -- a cut 'n paste from one of my other posts on this subject:
It's all in the chemistry. Many of the compounds in those additives, such as zinc dialkyldithiophosphate are already in motor oil, so you're just adding more of a good thing, that can lead to deposit formation on your valves, spark plug fouling and deterioration of catalytic converters.
Some of those additives may "help" motors that have ALOT of miles and have low compression -- like a "band aid". But I don't see the need for their use in healthy, well maintained engines.
Slick-50 and the other additives that contain polytetrafluoroethylene (or Teflon) is a solid that coats non-moving parts like oil passages and filters. If it can build up under the pressures and friction exerted on a cylinder wall, so it stands to reason it should build up even better in places with low pressures and virtually no friction.