Welcome to the forum. This question has been one of the most talked about subjects concerning the Crossfire, and by surfing the site you will see it mentioned alot.
Personally, I've been able to find the oil at Wal Mart some time, but not always. Auto Zone usually has it in stock. Price is never cheap, always $4.00+. a Qt. The Factory oil filter (the only one I use) costs around $21.00 at either the Chrysler dealer or the Mercedes dealer. If you are at Auto Zone pick up an oil filter "B" Cap wrench, just a couple of dollars. Changing the oil yourself will save you lots of cash over the dealer changes, and is really easy to do. It only takes a couple of minutes to drop the "belly" pan that protects the underside of the engine. I don't even have to jack my car up. I simply pull the front tires up on a couple of 2X10" wide boards and that allows me enough room to reach everything. Just be careful to get all the o-ring seals back in their proper position on the oil filters bayonet. Good Luck