CitriStripper to remove the cosmoline and old finish. CitriStripper is a furniture finish remover, so it won't h hurt the wood. It will probably take more than one application. I often scrub it off with a soft brush and hot water in the shower. If the wood is walnut, I may use some crystalline Oxalic acid wood bleach, which does a good job of lightening the the wood and high lighting the grain. If I want it to get the look of old aged stocks, I apply one, or two, coat/s of Raw Linseed Oil(Flaxseed oil is the same thing, just food grade). Followed by one, or two, coat/s of Boiled Linseed Oil, or Pure Tung Oil.
The Linseed Oils will age and turn the dark reddish brown that one sees on old military stocks. RLO will turn color a bit quicker than BLO. PTO does not change color.
If the stock is Birch, and has some figure, I may use a thinned stain with some red in it. Usually applied after I have one coat of finish on the stock.