There's no point discussing it with Amex - nothing good can come from that (although bad things could). It codes the way it's gonna code, and nothing you say to Amex (or Visa, for that matter) will change the way it codes. And that's not just for gas stations, it's how all payment systems work. Just because Visa/MC codes a place one way does NOT mean Amex codes the same store the same way, they often do not IME.

Move on.