We had a cooking oil spill on our unfinished basement staircase. Tried everything possible to remove the oil, but it had quickly soaked into the wood. My local paint supplier suggested an oil based stain that would cover the damage and a water based low luster floor poly. After an aggressive sanding the oil damage remained but the stain covered it perfectly. I did 3 coats of poly with sanding between. Couldn't be happier with the results.
Cooking oil soaked in visible even after an aggressive sanding.
Modern Walnut oil based stain and water based low luster floor poly.