Redex is probably the best known fuel additive (and is available everywhere from Halfords and filling stations to supermarkets). Most people who use it praise it but the same probably applies to homeopathy and witchcraft; whether the effect is real or 'all in the mind' is still open to debate.
A good hard run clears all the C R A P Hammerman, I use Wynns Redex every 2>>3 Fill, do not use cheap otherwise you will pay the price, if the van is Black Smoking replace the Injectors.