I'd think that a degreasing soap followed with a methanol rinse might help, but if it has sunk into the surface you might try plaster of paris. Mix plaster of paris with water to a paste. To each cup of the mixture you make, add one teaspoon of degreasing dishwashing liquid and mix. Apply the plaster of paris over the stains, allow it to dry completely, and then sweep it away. It should suck the oil of the concrete.