Wear rubber/latex gloves! No strong rubbing during any of this cleaning or you will damage the surface.
Take a cheap plastic wall paper tray, heat the ends with a heat gun to stretch out the ends to make room for a long SKS barrel, pour in a can of Oderless Mineral Sprirts. Disassebly weapon placing only metal parts into tray. Soak overnight or a couple of days, if needed. Once Cosmoline is desolved or soft use an old tooth brush or other small brush to clean off the remains. Tooth pick in the corners.
If metal is brown from surface rust use 0000 steel wool soaked with oderless mineral sprirts and gentally , DONOT push down hard .You use any force and you risk damage to the metal treatment.
On the wood use a half oderless mineral sprirts / half tung oil mix and a fine pad to remove the cosmoline and put some oil into the wood. After cleaning you gould use a fine sandpaper to remove any humidty swelling the wood experianced over time and not damage the original look, just don't get carried away, then everyone will notice the finish is too smooth.