FURNITURE TIPS - Removing oil from a wood surface.

The only way to pull grease and oil out of wood is with a poultice. Go to your swimming pool place and get a bag of diatomite or commonly known as diatomaceous earth. Grab a couple of gallons of acetone also from hardware or paint store. Mix the two together to form a paste you can spread over the top. You will need about a 1/2" layer of it. As the acetone dries, the oil gets pulled into the DE .Once it is dry, scrape it off into the trash and repeat if necessary. Most likely it will take at least three applications. You might try putting a heat source directly under the slab at the same time which will drive more oil to the surface.
You will need to do it in outdoors because of the intensely flammable and highly smelly fumes.