I have used ashes for a while now to soak up oil. They are free based on my grilling. Once I have soaked up the oil I then use straight dawn dish soap on the stain, scrubbed in with a brush or broom. I let it sit for a long time, like all day, then I power wash it off. If anything is left behind I will rinse and repeat or use brake cleaner to finish it off.
Just my $0.02