my trunk interrior is filled with mobil clean 5000 motor oil for months. i cant seem to take it off, i wiped it with towels, newspaper, etc. its pretty much soaked in oil, and i just put newspaper over it to put stuff in the trunk
I always have spilled oil in the hatch of my civic due to hauling around motors all the time with it, and engine degreaser or a powerful garage-duty mutlipurpose cleaner should take the residue off. Just be careful since some degreasers are powerful enough to melt plastic or stain carpet.
On the never-ending quest for better gas mileage...