A few years ago I cleaned out my garage and realized how much dirt and motor oil had been ground into the concrete floor. I made a mixture of dish soap and water and used that to try and scrub the floor clean. Unfortunately this did next to nothing. Eventually I went out and bought a product that was made specifically for cleaning concrete stains. After I used it there were still some faint stains from the motor oil but they were a lot lighter than before. Unfortunately I can't remember the name of the cleaning product but it was pretty easy to find in the hardware store.