You need to use a really strong solvent designed to remove oil. (Ask Home Depot). How old is the stain; if it is relatively new, put some fine sand on it to absorb as much as possible before cleaning Bottom line, it still may show no matter what you do. If it does, think about staining (not painting) but staining the concrete to mask the stain. Unless you want the pure white look, staining can add some character — you either use 1 color, or can use a few different ones for a mottled, stone like look. You can then put a clear sealant over the concrete that would repel oil in the future. Good luck!