Have you tried Meriatic Acid? Just pour it on, let it stand 10 minutes and rinse it off.
At Home Depot you can get a product called Krud Kutter. It takes grease out of everything but I've never tried it on concrete. Painters use it for clean up and spills. You might try it - it's highly concentrated and I use if full strength all the time.
It's wonderful stuff. Someone once told me that Pert shampoo would take oil stains out of concrete as well.
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