If you don't like the dynamite, Pick and Shovel followed by a Big Ole Patch ideas you can always park on the grass and walk in thru the back door.
On a serious note though, maybe a heavy brush, simple green, hot water? Pressure washer? Don't think you'll do any good though. My Cummins marks it's territory also, I just forget about it, park in different locations everyday this way I have an equal stained look.