Product to remove oil stains from concrete for painting?

I work in the monument (gravestones) industry and we occasionally need
to remove oil spots on granite/marble monuments, usually caused by
careless lawn workers putting an oil can on a monument.

While the following method may not be suitable for an entire floor, it
works good for smaller spots. Make a poultice paste of common white
plaster and laquer thinner (or white gas) and slather it on the stain
with a trowel, leaving it approx 1/2" thick (thickness not critical).
As it drys the plaster will draw out the oil. When dry, scrape/sweep
it off. Be careful not to let the plaster get wet from water (if
outside cover with plastic sheet) or it will harden and will have to
be chipped off with a chisel. Like I said, this may not be practible
for a large floor....but will do the trick for smaller stains from
engine oil drips on a hanger floor. Deep stains may require more than
one treatment.

Usual disclaimer about being careful with fire/sparks near flammable
substances, etc.

Rich A.