Actually, the best way I have found to remove oil stains is kitty litter - the cheap stuff. You could use oil dry, but it costs more. Spread a generous amount on the stain and grind it in with your foot (please wear a shoe!) until pulverized and the consistency of talcum powder. Sweep it up and do it again. This should get most stains out. (The clay, when fine enough, is pushed deep into the pores of the concrete and literally sucks the oil out.)