Removal of a grease stain on untreated wood
Rub the stain with a brush soaked in lighter fluid.
Lemon juice, white vinegar and bleach can also work.
Once the stain is removed, wash the area with soapy water then rinse.
Removal of a grease stain on polished wood
Apply a mixture of diatomaceous earth (or diatomite) and White Spirit on the stain. Dry and brush.
You can also cover the stained area of polished wood with a thick layer of talcum powder or diatomaceous earth (or diatomite), then apply several layers of brown paper and finally pass a hot iron over the paper. The paper will absorb the stain.
If all else fails, sand the wood down and then polish it again.
Removal of a grease stain on varnished wood
Rub the stain with a sponge sprinkled with baking soda.