I would use whatever concentration you used before and clean the rest of the asphalt in the same manner. At least, that way, you'll have a better chance of it all being a like color again. Then, I'd get a sealer and apply it to the asphalt according to directions, using as many coats as necessary to accomplish a good seal.
In future, I'd make sure I had oil guard pans or sheets of heavy cardboard, (that I changed often), something that would help prevent any future spills, leaks, etc. from discoloring the asphalt.