Don’t apply dichloromethane and methanol it is very dangerous. Put a little household detergent onto the sandstone (being sure to follow the manufacturer’s instructions in case you need to mix it with anything) and scrub with a hard bristled brush in a circular motion until you can no longer see the oil stain. If older then use bleaching powder.you can also use mixing acetone with baking soda.

Prem Baboo Thanks for your suggestions! This is a reservoir rock, so the oil is within the pore space as well.

For sandstone porosity oil removing you can apply CTC, Acetone or solvent for oil, then steaming for removing dirt.

It is more preferable to use warm water to soak the sand stone in it especially the part containing oil then later use a towel to absorb the water with the oil.