Kerosene, carboreator cleaner, gas, anything like that will clean the cosmo off the metal but the stuff that has soaked into the wood stock is another thing. I use Murphy's Oil Soap and it does a really good job of getting the excess off. You don't want to get the stock too wet or it might warp a bit on you. After I have gotten the excess off and the stock looks clean, a couple of range sessions that warms up the action and barrell usually gets the remaining cosmo to leach out of the stock.
One of my old Mausers was soooo soaked with cosmo that even after the Murphy's I put the stock under a couple of heat lamps for a day and went back every couple of hours to wipe it down again, and the cosmo is still not all out. Mikey.