Auto Zone is presently running a special detailed in its' January 15-February 11, 2009 in store flyer - $2 off Rotella Synthetic 5W40 gallons regular price $19.99 so its cheaper than Walmart and probably closer. And better still if you have an Auto Zone rewards card:

"From the rewards website:For each single in-store qualified purchase of $20.00 or more, you will earn one (1) Credit. For every 5 Credits earned within 6 months, you will receive a Reward good for $20.00, which can be used for purchases made at a participating AutoZoneŽ store."

You can stock up and after spending $100(5x$20) you can get a $20 gift card! Or another way of looking at it is another almost $4 off the original purchase of a gallon of $17.99 oil and a $2.01 item (maybe an inline plastic fuel filter, matching quart of oil, etc.)! Good luck!