Those with an American Express card  get 5% back on top of your regular card rewards to fill up at any Exxon or Mobil station between December 20, 2010 and January 20, 2011.

Here’s the promotion link.

From the site:

To be eligible for this offer, you must be the named recipient of this invitation and register any eligible American Express Card by January 19, 2011 at americanexpress.com/exxonmobil. You may register more than one Card for this offer, but only those Cards you register individually will be eligible for this offer. Using your registered Card, you must make at least one purchase at any Exxon or Mobil Station between December 20, 2010 and January 20, 2011. Prepaid, American Express OPEN®, and corporate cards, and ExpressPay transactions, are not eligible. If your Card is replaced during the promotional period, please call the customer service number on the back of your Card for assistance. Statement credits are generally issued within 5 business days upon completion of the eligible transaction(s), but may take up to 2 billing cycles to post to your account. Additional terms and conditions apply. See Registration Terms and Conditions for details.