Tony,
Sounds like a job for a pressure washer underneath. Engine
oil leaks can really mess up the underside of a car. But you'll find that the Mirachem works. For example, an area I had a hard time with was the white fiberglass on the footboxes on my ERA. Surprised me how quickly the stuff cleaned it up. Same applied to the cast-aluminum rocker covers. Made them look new.
Waiting to hear how it works for you.