Ruger has always stated that it was an oil finish. That said, Tru-Oil rubbed in and rubbed to a satin finish using 0000 steel wool is the closest I've come to duplicating it. Modified tung oil works pretty good too. I've used Formby's furniture finish, Danish Oil Finish, and several other brands of modified tung oil. The key is to applying repeated finishes to completely fill the pores of the walnut, with couple days drying time between coats and sanding it level using 600 wet-or-dry auto finish sandpaper.
