I read the fine print of the AMEX coverage, and such a thing (although it falls into the category of negligence on the part of the renter) doesn't seem to be excluded. The AMEX documentation states that "Damage" is defined as "the effect of any contact with or treatment of the rental car which requires repair in order to restore the rental car to its pre-possession condition". The list of "excluded actions" has nothing that would seem to preclude the accidental tanking up using the wrong fuel.