Oil stains are almost impossible to get out of concrete. Substances that tend to sit on the top surface of the concrete are much easier to get off. We did some testing with various stain and graffiti removal products. Nothing worked perfectly.
Since this show I have been looking for something that actually works and finally found a little known Canadian made product that not only works, but it is bio degradable. It is called OIL LIFT and can be found in auto supply stores like Canadian Tire.
An important tip with respect to auto oil stains on driveways, soak it up a soon as it happens with something like cat litter so that there is simply less to soak in. When cleaning, work from the edge of the stain to the centre to avoid spreading the problem.