A fresh stain, still dark and wet looking
Oil that just hit the concrete is still mostly sitting on top of it, which is the easiest stage to deal with. Covering it with an absorbent material and leaving it alone does most of the work before you ever pick up a brush.
- Cat litter, baking soda, or cornstarch pressed into the stain and left overnight
- Sweep up the absorbed material once it has drawn the oil up
- Follow with a concrete degreaser scrubbed in with a stiff brush, then rinsed
An older stain that has already soaked in
Oil left on unsealed concrete for weeks or months has had time to work its way past the surface, into the pores of the concrete itself, which is why a driveway degreaser alone sometimes only lightens an old stain instead of erasing it. Concrete is more absorbent than it looks, especially where it has never been sealed.
The same degreaser treatment still applies, worked in and given time to pull the oil back up before rinsing, and it often takes more than one round on a stain that has had years to set in. Patience does more here than a stronger chemical does.
Why a shadow can remain even after cleaning
A stain that is 80 or 90 percent gone but leaves a faint gray or dark patch behind is common on an old, unsealed slab, because the deepest part of the oil sits below where scrubbing reaches. That is not a sign the cleaning failed, it is a limit of how far a surface treatment can pull oil back out of porous concrete.
A broom finished driveway, the most common finish for one, has more surface texture than a smooth troweled floor, which gives oil more area to work into as well. A pressure washer speeds up rinsing a degreaser, but it does not replace it, and running one alone on a set in stain mostly just moves dirty water around without lifting much oil.
Keeping oil from soaking in again
A sealed driveway resists oil staining far better than a bare one, because a sealer keeps oil sitting on top of the surface instead of soaking into it, which makes the next spill easy to wipe up instead of a project. If a spot under a parked car or a mower is a repeat problem, that is a reasonable question to bring up next time we are out.
In the meantime, catching a fresh drip with cardboard or a drip pan under a vehicle that leaks does more to prevent a stain than any cleaner does after the fact.