Direct answer: an oil stain younger than about a month usually comes out with an alkaline degreaser, real agitation, and a hot rinse. Anything older has soaked into the pore structure and needs a microbial cleaner or a poultice to pull it back up. A deep multi-year stain often never fully leaves.
Cured concrete is not solid. It is roughly 12 to 18 percent void by volume, laced with capillary pores running from about 0.01 to 10 microns. Motor oil has a surface tension near 30 mN/m against water's 72, which means oil wets concrete better than water does. That single fact explains why rinsing never works: water cannot displace something that clings to the pore walls harder than water does. Fresh oil wicks about 1/16 to 1/8 inch down in the first day. Left alone on unsealed concrete through a season of hot afternoons, it migrates a quarter to a half inch deep, and the dark patch you see at the surface is the top of a cone. So the real question is never what product to buy. It is how deep the oil went, and whether your method reaches that depth. Degreasers reach roughly the top 1/16 inch. Nothing else you buy at the store does.
If the spill is still liquid, absorb before you clean. Cover it in clay cat litter or oil-dry, grind it underfoot to break the granules open, leave it an hour, sweep it up. Do not hose it first; water spreads the slick and drives it into pores. For a stain that has set days to weeks, use an alkaline degreaser in the pH 11 to 13 range. Alkali saponifies oil, converting it to something water can carry. Wet the slab first so the product stays on the oil instead of soaking into dry concrete, apply, and agitate with a stiff nylon deck brush. Nylon, not wire, which leaves rust flecks. Dwell 10 to 20 minutes and never let it dry, misting if needed. Work in shade or after 6 p.m.; a slab above roughly 90°F flashes the product off before it does anything. Rinse hot if you can. Expect two rounds.
Past a couple of months, surface chemistry stops being enough, and you have two tools that actually go down into the concrete. Microbial cleaners, sold as bioremediation or oil-eating products, seed the slab with bacteria that metabolize hydrocarbons. They are slow and unglamorous: apply, keep the area damp, wait 24 hours to a week, repeat two to four times. They run $25 to $60 and are the best value in this entire guide for a stain one to three years old. A poultice works faster on a concentrated spot. Mix an absorbent, diatomaceous earth or hydrated lime, with acetone or xylene into a peanut-butter consistency. Spread 1/4 to 1/2 inch thick, extending an inch past the stain edge, cover with plastic sheeting taped on all four sides, and leave 24 to 48 hours. Then pull the plastic and let it dry completely. The drying is the part that pulls oil up. Scrape, repeat two to four times.
Pressure washing removes what chemistry has already loosened. On its own, against oil that is in the concrete rather than on it, high pressure mostly just etches the slab. That is the most common expensive mistake we see. A zero-degree red tip at 4000 PSI will strip the cement paste and leave permanent striping shaped exactly like the arc of your wand, and the stain will still be there underneath. Use a 25-degree tip at 12 to 18 inches, or better, a rotating surface cleaner at 3000 to 4000 PSI and 4 GPM, which spreads the load evenly. Flow matters more than pressure for carrying oil off the slab. Hot water is the real advantage a professional rig has: 10W-30 loses roughly 90 percent of its viscosity between 70°F and 180°F, so heated water emulsifies what a cold homeowner unit at 1800 PSI just pushes around. Never pressure wash concrete under 30 days old.
Some of this does not come out, and knowing which before you spend saves real money. Automatic transmission fluid and some gear oils carry red dye that penetrates and stains independent of the oil itself; you can remove the grease and keep the pink. Brake fluid, a glycol ether, strips sealer and can leave a permanently lightened patch. If the concrete around the stain is pitted or crumbly, the paste is already gone and cleaning only exposes more aggregate. And the one nobody warns you about: on a 15-year-old driveway, cleaning a single 3-by-4-foot spot leaves a bright rectangle that reads worse from the street than the stain did. If a stain has fed a leak for five-plus years, plan on 40 to 60 percent improvement. One to two years, 70 to 90 percent. Anyone promising complete removal on old oil is guessing. When it truly cannot be fixed, the remaining options are a polymer overlay at $2 to $5 per square foot or replacement at $6 to $12.
Do not pay anyone to pressure wash one oil spot. If it is a single stain on a driveway you like the color of, buy a $30 microbial cleaner and give it two weekends; you will get a better result than a truck can deliver in an hour, for a tenth of the money. Hire it out when the whole slab needs washing, because then the cleaned stain blends instead of standing out. Reflekt prices pressure washing at $0.20 per square foot, so a typical 600 square foot two-car driveway runs about $120, and 900 square feet about $180. Locally, plan the work between April and October. Below 40°F, wash water freezes in the pores and does damage of its own, and the valley puts a slab through roughly 100 freeze-thaw cycles a winter already. Keep degreaser runoff out of the street; valley storm drains discharge untreated to the Boise River. Push rinse water toward landscaping or gravel.
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Absorbents only pick up liquid oil still sitting on the surface. Once oil has soaked into the pores, cat litter, baking soda, cornstarch, and flour have nothing to absorb, because the oil is no longer free to move. They are the right first step within minutes of a spill and useless a week later. For a set stain you need something that either converts the oil chemically or draws it back up out of the concrete.
No, and both cause damage. Muriatic acid dissolves cement paste, not oil. It lightens and roughens the surface so the stain looks fainter for a few weeks, then reappears as the newly opened pores let the oil below wick back up, now on a slab that is permanently more porous and stains faster. Bleach does nothing to a hydrocarbon at all. Skip both and use an alkaline degreaser or a microbial cleaner.
Only on a stain a few days old. Pressure removes what chemistry has already loosened, so without a degreaser or microbial cleaner doing the work first, high pressure just etches the concrete around the oil. A homeowner unit at 1600 to 2100 PSI and 1.5 GPM will not touch a year-old stain. Apply the chemistry, let it dwell, agitate, then rinse. The rinse is the last step, never the whole plan.
Reflekt charges $0.20 per square foot for pressure washing, which puts a standard 600 square foot two-car driveway near $120 and a 900 square foot three-car near $180. Measure length times width to estimate yours. Deep old oil that needs poultice work is a separate conversation and is not something to buy sight unseen. Call (208) 453-6760 and describe the stain's age before booking anything.
Sometimes, but set your expectation by age. A stain under a year usually goes to near-invisible. One to two years, expect 70 to 90 percent lighter. Five years of steady dripping under a parked car, 40 to 60 percent, with a shadow that stays. Dyed fluids like automatic transmission fluid can leave color behind after every trace of grease is gone. Concrete that has spalled around the stain will not go back to uniform no matter what you use.
Yes, once it is genuinely clean and dry. A penetrating silane or siloxane sealer fills the capillary pores so the next drip sits on top long enough to wipe up. Wait 24 to 48 hours after washing so the slab is dry all the way through, and never seal over residual oil, because the sealer will not bond and will peel in patches. Reapply every two to four years in this climate. New concrete needs 28 days before its first sealing.
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