Heating the cosmoline? It will make it very hard, and hard cosmoline is an extreme PITA to clean [i've actually seen the driving shaft in a drill-press get snapped once while trying to un-seize it; the culprit? long storage and hardened cosmoline]
Make sure you disassemble your bolt and thoroughly clean it out, especially the firing pin and its area. (many stories of SKSs which haven't had this done have gone slamfire/fullauto after the cosmoline got hard and jammed this firing pin).
I'd recommend a very thorough detail-strip, cleaning everything with a good degreaser, then liberally oil the whole thing up. Use those pipe cleaners in hard to reach places, like inside the bolt, gas port, around the front sight, inside the mag-catch.....