The Handyman's Pocket Book: Cleaning An Oilstone.

It is a good plan to wash an oilstone occasionally with paraffin to get rid of any old dried-up oil.  If the surface has worn out of truth it can be corrected by rubbing down on a flat paving stone or piece of marble.  It should be fed with an abrasive of sharp sand and water - unless the stone is of the synthetic type, in which case corundum powder should be used.