I've never understood exactly what these alleged cleaners do... surely fuel filters are there to stop bits from blocking the fuel pump/injectors, etc? And it can't possibly help with the IMRC/EGR sludge, as by that point it's already been burnt, so will have lost any cleaning properties it may have originally had.
Can anyone actually explain how these additives clean the engine? I don't doubt that engines seem to run better when you add the stuff, as they also have octane/cetane boosters in them, which will obviously perk up your engine for a while (maybe a few fill-ups?). The instructions do also hide behind weasel words when explaining the benefits (e.g. 'may help', 'should aid', 'many satisfied customers claim that...', etc) which makes me a bit suspicious!