I was taught that I could get oil and other stains using a verity of paints and additives. I mix different ratios of burnt-umber and black artist oils along with chocolate Humbrol enamels. I also add rust colored pigments and gloss varnish. Mixing these mediums at different amounts will give you both new and old looking stains. Thin the stains using turpentine or Humbrol thinner.
I hope this helps