Fuel makers claim that their premium fuels include superior detergents compared to their ordinary fuels. They might, of course, be deceiving. However, how can one ascertain with certainty the contents of a bottle of Redex, Millers, or Whyns? They may be repackaging standard petrol or diesel and marketing it in little bottles at exorbitant costs as 'fuel system cleaning'. I am unable to substantiate this assertion. If you, as a consumer, lack confidence in the veracity of the manufacturer's assertions, refrain from purchasing their items - it is your prerogative.
b. MB does not endorse the use of gasoline additives introduced by the driver. However, they do not restrict the use of fuels that have had additives included by the gasoline vendor. The owner's handbook for my W203 said so, but I am uncertain whether Mercedes-Benz's recommendations have since evolved. Consequently, the use of premium fuels aligns with MB's recommendations.