Through 12/31, Amex has teamed up Exxon and Mobil and you can get 1 additional Membership Rewards point per dollar spent on purchases made using your ENROLLED Card at US Exxon or Mobil locations.
If you have an American Express card that earn Membership Rewards points, you likely have this offer available to you as well. Login into your account and check the offers at the bottom of the page.
One last thing, before you think about getting your car serviced/repaired at an Exxon or Mobil location and earning 2x points (what I immediately thought of), know that this offer really just applies to gas purchases. Specifically excluded in the offer are the following: purchases inside convenience stores, car washes, automotive repair, garage services, etc.