I feel it’s my duty to contribute to this old thread – for completeness.
Basically, if you use supermarket fuel and only drive short distances the a horrible black glue-like sludge will start collecting in your EGR and associated pipes. Also, your turbo will also become faulty and eventually fail due to the variable power fin system becoming stuck/restricted/blocked.
I have been shown/have physically seen engine parts, removed from modern VW engines which have suffered this fate, by an audi master tech.
I support of the replies describing the ‘state of the art’ additives – YES. This is correct, it’s not really an octane ‘Boost’ OK, it’s (as has been described) keeping the engine clean and efficient – so it’s does not die of the dreaded ‘Black-Death’.