What I've used on really dirty Trapdoor Springfields is a mixture of half and half linseed oil and turpentine, soft, lint free cloth and an old, soft toothbrush. It does a good job of removing really bad grime without injuring the patina and it adds a bit of linseed oil to old, dry wood. For really bad spots I have also used some OOOO Rotten stone with the mixture. Be gentle and do a little at a time. For a varnised finish old Model 70 I suspect Alvin's suggestion might be the best.

Jerry Liles