Thought zx1 was a crappy computer in the 80's?
there was quite a thread about engine additives in non-transit talk called
Oil treatment stuff, is it just a gimmic ??????
type 'slick 50' into the site search and you'll find it (im too stoopid to post a proper link)
the upshot was that A) although expensive, some people swear by them B) they're no substitute for proper maintainence and C) some people believe they can actually damage an engine by blocking 0il filters.
It seems a bit of a Marmite question- you love em, or hate em. Personally I did use Slick 50 at my last service, but probably wont bother next time.