I used to be involved in this kind of work in an industrial setting, however, my knowledge is 20 years old, so there may be other options today. What the EPA rules were at the time was to take up the first 6 inches of soil and send it off for disposal. Test the soil at that depth and if no oil, then fill back in and done, but if testing showed more oil, then dig up another 6 inches and send off. Keep doing that until you get clean samples. Gasoline was a different matter, dig up 6 inches and place on a heavy plastic tarp and stir soil regularly, the gasoline will evaporate and then you can just replace that soil back.