Start with something cheap (like the brake cleaner someone suggested) and plenty of rags. Then move onto commercial gun cleaners like Hoppes to finish things off. You should do the barrel last and pay close attention to scrubbing it well to get out any stray metal chips. For the barrel, I would finish it off with Sweets 7.62 to get out all of the cosmoline greasy kid stuff. Then oil her up.
the "old school" way taught to me by a WWII vet of cleaning cosmoline was to use kerosene or even gasoline. but for obvious reasons you need to be real careful and i would not recommend gas at all. these days i second the brake cleaner solution. break the gun out of the wood and into as many peices as you can put back together properly and soak it good in brake cleaner and then use an old toothbrush to get it all off. depending on how well you can break it down, you could then boil it in hot water with some dawn dishsoap and scrub it real good. then let it dry thoroughly and spray liberally with rem oil and then wipe it off. stand all parts vertically on an old towell for a day or so to let the excess run off and then wipe it again and let it stand some more. when it doesn't stain the towell anymore put it back in the wood.
The "out in the sun" method works....put it on a towel on the back dack of a car in the strong sunlight and it'll melt. Works better in the summer, though.
I use my wife's hair dryer. Heat it carefully (you won't damage anything, but you might make it too hot to hold!) and watch the cosmo drip! Then clean with alcohol, then repeat if necessary. Trust me, you'll sweat cosmo out of wood LONG past when you think it's all gone. And the heat will get it out of the nooks and crannies that nothing else will. I degreased and M1 Garand and a Mosin-Nagant both this way. Works like a charm.
Gasoline/mineral spirits is what I've always used. Outside, away from any ignition source and on a gravel limestone driveway.
Gasoline and a stiff parts cleaning brush, to remove most of it quickly and mineral spirits to finish up with. Used brake cleaner aerosols to flush out the trigger, magazine and bolt assemblies, after initially soaking/brushing them with gasoline.
Last rifle I de-gunked, was an Inland 30 Carbine from CMP, also did an SMLE and a 1969 Romanian 22 trainer. Even the wood was heavily-coated in preservative, on all three, but really bad on the SMLE.
After I'd removed the gunk from all the parts, left the stock pieces out on the deck in the sun, on newspaper. Took repeated cleanings to finally get the stock to the point where it was no longer sticky.
I would <u>not</u> advise anyone using gasoline, unless they can do it safely, as mentioned above in the first paragraph.