First off there is a lot of good info offered and some not so good info offered so far.
You have to look at it this way. You are going to be cleaning two different types of material, wood and steal. You can't clean them the same way and expect to get the same results.
1st, like Boom Boom said, remove as much of the cosmoline with a plastic scraper, or wood Popsicle sticks and rags. If you have a small jar, save the cosmoline, You may be able to sell it to a collector one day.
With the gun dissembled use boiling water like cjwils said. I like to hand the action using wire over a metal container that I put the small parts in. To speed up the cleaning you can use a cleaner, but it's best to use one that will work with water.
I like to use Tilex Bathroom cleaner.
I use a tea kettle to boil my water. While I'm waiting for the water to boil, I spray down the metal parts with the Tilex. Once the water is ready just pore it down and over the barrel. Spray on more cleaner and then more boiling water. This should get just about everything off the metal.
Pore the water out of the container and lay your small parts out to dry while they are still hot.
Apple a good gun oil to all metal parts while they are still warm and let them sit over night.
The next day just scrub the parts with a nylon brush as you would with any other gun you would clean.
Now for the stock. Remember that the oil and cosmoline did not soak into the stock over night and that you will not be able to get out of the stock over night. You will be able to remove the oil and cosmoline from the surface and some from just below the surface.
You can use the Tilex to clean the wood, as long as you do not let it dry on the stock.
Oh! the handguard can be a pain to remove. you can just leave it on the gas tube and clean it with the stock.
Spray the stock down with the Tilex and use a soft scrub brush to scrub the stock. Use the water hose to wash down the stock and then dry off with a rag or paper towels.
DO NOT HEAT OR SET OUT IN THE SUN TO DRY!!!!!! This can cause trouble like the stock warping.
Put the stock in a cool dry place and allow it to dry. Your stock will be lighter in color once it has dried, but you are not none yet.
Now it's time to apply heat to the stock. You don't want to get it too hot so, be careful if using a heat gun. I like to set it out in the sun and let it sit for a few hours or more..
Oil will come to the surface. If it's a lot, put paper towels into the action area and cover the stock with some more paper towles then wrap with news paper. Set it out in the sun. The back dash of your vehicle works great to heat the stock up on a warm day , but your wife may not like the smell.
Wash and dry the stock like before. You may have to wash it 2 to 4 times.
Do not sand the stock. It is most likely made of Chue wood and will fuzz when sanded.
To smoothen the stock all you have to do is Bone it. This is done by rubbing the stock down with a hard wood dowel. This will compress the and smooth the surface.