Go over to the milsurps forums such as parallelx and read up on what they call stock cleaning. Several methods are available for removing the cosmoline while not destroying the original finish. You can also touch up the original finish to preserve your wood stock.
Many collectors do not want an original stock changed. But many original stocks have damage, be it oil or physical and need maintenance/repair. Not maintaining the stock will at some point end up in that stock's failure. Cosmoline or oil soaked into a stock can and will destroy the wood fibers of the stock over time, causing the stock to fall apart, shrink, become soft, etc.
Here is a couple links to what I consider to be reputable surplus rifle forums:
http://parallaxscurioandrelicfirearmsforums.yuku.com/directory
http://www.surplusrifleforum.com/index.php?sid=2f2b2b1b3d916c44fdc2cb05e7443be7
Both of these forums have very good stock care forums in them. I find the methods used at parallax board to be preferred for the preservation of the stock.
BTW, once upon a time I bought a Swedish Mauser with a stock that looked like three miles of bad road, totally oil soaked and looked like it was about to fall apart. All the colllectors advised against cleaning and/or refinishing. But looking at the stock, I realized it was on it's last legs and would soon fail. I stripped the cosmoline/grease/oil out of that stock. Once clean, the wood was still in good shape, but the finish had been destroyed by the cosmoline. I researched, found out the original finish and replaced the stocks finish. I was careful and resisted the urge to use sandpapers and instead used 0000 steel wood to do any "polishing" that needed doing to clean up splinters, rough areas, etc. Once done, the stock looked beautiful. I kept it for a couple years, shot it a time or two and sold it for my full asking price at a gun show. The show started at 8 AM in the morning. The rifle was gone by 8:30 AM. Two collectors came by and told me how I'd ruined the stock and how it was now worth no more than a sporter stock. Then a guy that wanted a Swedish Mauser came by. He picked the rifle up, looked it over, pulled out his wallet and bought that rifle and two others I had for sale (An Enfield #1 MkIII* [rebuilt from a barreled action] and a M95 8MM carbine [in exc/like new condition]). He paid cash, paid my full asking price and didn't even act like he was interested in dickering.
The moral of the story is: Some folks don't want anything restored, some folks love a fully restored original. Kinda like used cars. Some want factory original, some want restored factory original and some want crazy hot rodded old school cars. Myself, I love an exc/like new factory original, but I also love a properly restored (looks like it was rearsenaled) military surplus rifle where a rough gun was brought back into fighting trim and ready to go to war.
) $300-400 ish depending on market and ammo availability if all numbers match and the metal is in as good condition as the stock. A tad less for a mixmaster shooter. Throwing some 8mm ammo into the deal might assist in the sale. (FWIW)
