Use equal parts of Fullers earth and water and mix to a paste...then add a small amount of Trisodium Phosphate.( EBAY) Spread it over the stain and leave for a few days.Depending on how deep the oil has penetrated it may need more treatment.
just as a point of interest, is the floor bare concrete at the moment ? one of the downsides of painting garage floors is although they look pretty come winter when you park in it (assuming you are using it as a garage) all the snow and ice melts and lays on the paint creating a really wet & slippery surface.