Try some laundry detergent. Work it in with a stiff brush, let it sit for 15 minutes, and rinse. There are also degreasing products available at better home centers and hardware stores.

You might also want to fix that leak. Oil leaking from cars is major source of groundwater contamination.

Thanks for the info Mike. I'll give it a try.
thanks too for the insight about the leak, I never thought about it contaminating the ground water.