Q.: What is the best way to remove motor oil stains? The oil has probably soaked pretty deeply into the concrete.
A.: Either mineral spirits or a strong water solution of a detergent (1 part liquid detergent to 5 parts water) can be used. But a good result cannot be obtained simply by applying these materials by themselves to the concrete even with scrubbing. The problem is that they carry part of the oil even more deeply into the concrete.
The mineral spirits or the detergent solution can be used, however, to make a poultice that can be plastered onto the surface. Fly ash, hydrated lime, or other inexpensive finely divided material can be used with mineral spirits or the detergent solution to make a paste to be used as a poultice. Shredded newspaper, paper pulp or sawdust are also useful. As much liquid should be used as needed to make the poultice trowelable or moldable. The product is applied to the surface and left in place.
The liquid slowly migrates into the concrete and dissolves some oil. Meantime, as the poultice begins to dry, the dissolved or emulsified oil begins to move out of the concrete. As the solvent or water evaporates, it leaves its load of oil in the poultice. When dry, the poultice can be lifted off the concrete and discarded.
The advantage of the detergent is that it doesn't produce objectionable fumes and can be used where solvents are banned. But it takes more treatments to get the same result as with mineral spirits. Mineral spirits must not be used without excellent ventilation and care must be taken to prevent explosions or fires caused by sparks (for example from an electric fan).