I'm not sure the exact rate the Amex uses. Based on their T&Cs, they say:
"We use exchange rates from customary industry sources" which could mean MC, Visa, or any other "industry source".
Compare this to BW that say in their T&Cs:
"foreign currency transactions are converted from foreign currency into United States dollars and then converted into Australian dollars by Mastercard International Incorporated at a wholesale exchange rate selected by Mastercard International Incorporated on the processing date"
So BW unequivocally uses the MC rate, but Amex purposefully keeps it obscure.
As for Diners, they 100% do not use the Visa or MC rate. Their T&Cs stated:
"the amount of the charge will be converted to Australian Dollars at the rate of exchange determined by Diners Club or Diners Club International or its settlement agency"
So again, not every institution uses Visa/MC rates. The only way you can be sure they do is if they explicit tell you in their T&Cs or you get confirmation from the bank rep.
