what about "Magic Eraser", you find it in the detergent/soap aisle in the grocery store.

 

Wal-mart, K-mart, lowes, pretty much anywhere sells the stuff

 

Try going to an Autobody shop supplier and getting a product called Prep-sol. Its a wax and grease remover for when your going to be in the process of priming and painting a car.. If that doesnt work then time to get that area of the truck resprayed...also how long has the oil been on the door?