No matter how hard you try the cosmo and oil will never completely come out. You can stop it from coming out when the stock heats up though. This writeup is from Cabitman over on Parallax's site....
Spit Coating as a Cosmo Killer
There is another reason to spit coat, however. Many of our rifles come packed in cosmolene and that thin grease will seep into a rifle over many years, and become virtually impossible to remove, no matter how hard you try or what technique you use. Even if you think youve gotten it all out and apply the perfect finish to a stock, the first warm day or after extended shooting, cosmolene will being to weep out onto your hands. This can occur for years afterward. To virtually eliminate that, a barrier can be applied to the stock after its cleaned that will seal in the old cosmo. The material we use is shellac.
You can purchase a spray can of shellac at any good home center or paint shop. Its a thin, clear, shiny material that is impervious to oil. That means that oil can not penetrate from underneath once its one the rifle.
The term spit coat means we spray one or two very light coats onto the cleaned rifle stock, making sure to cover all the surfaces. Now this will make your stock look like crap. Thats ok. Its ok because you next sand off all the shellac to the surface of the wood. Youre not removing any of the wood so dont be concerned. Youll use a 180 or 220 silicone carbide paper as mentioned in the first part of this post, and simply sand away all the shellac on the surface. This will only leave the shellac that has penetrated the open grains of the wood. Since the old cosmo can only get out through those open grains but the shellac is now sealing them, the cosmo is trapped. This will keep your rifle from sweating cosmo forever.
After youve spit coated with shellac, you can finish as you normally would. You can go ahead and oil your stock or, if you need to add color your stock using dyes or stains, do that and then oil.
I recommend Tung Oil Finish, not BLO for our old stocks. Tung Oil Finish is much lighter and has good driers and solids in it that will build quickly. Apply your first coat of oil using 0000 steel wool, rubbing it in. Do a good, wet coat, let it sit, and then wipe it down. Yes, there will be plenty of wood to absorb the oil as the shellac has only closed off the open pores.