The fuel card in each car is tied to the fuel type it needs. If you don't use the right fuel, the card won't work.
For most standard cars, E10 or Unleaded 91 are the appropriate fuels.
For Audis, we ask you use Premium 95 or 98.
To be absolutely sure each time, check your booking confirmation. The card holder that holds the fuel card also tells you which fuel to use.