I have done a considerable amount of refinishing on old oil-soaked and water/rust stained stocks. The oxalic acid will remove the almost black rust stains in wood caused by a gun that has repeatedly gotten wet. Wet wood against metal rusts and causes black wood stains in the wood it is in contact with. Picture a SXS with metal butt plate standing upright against the wall of in a wet duck blind. Repeated brushed-on applications of oxalic acid will get rid of the black rust stains on the wood's surface, but does nothing to remove oil from the oil-soaked areas of a stock. So, I doubt that it will do much to remove oil-based stain from a stock, especially if in end-grain wood. You may have to settle to remove what oil-based stain that you can with solvent and rags, then sand off what remains.