This article is from the Toyota RAV4 FAQ, by Ralph Becker with numerous contributions by others.
No, absolutely not, never.
Lubrication system additives are never necessary as long as you are
following the manufacturer's recommendation for oil change intervals.
Additives do not provide any protection or performance improvements,
and can in some cases cause engine damage or excessive engine wear.
The Federal Trade Commission <http://www.ftc.gov/> has settled with
the makers of Slick 50 for making advertising claims that were "false
and unsubstantiated." Follow this link for more detailed information.
<http://www.ftc.gov/opa/1997/9707/slick.htm>
 
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