Mineral spirits seems to be the most harmless way to clean out hardened (and dirty) lubricants. I found a project 16-gauge Model 12 this summer (from 1919, only 6lbs5ozs!) that also needed a thorough cleaning. Mine was disembled and cleaned, but I've been told by fairly credible sources that just soaking the mechanism (with even diesel fuel!) will work (M12s are practically indestructable). Remove the barrel and forend and just insert the action into the fluid of choice. I'd not soak the stock if you can avoid it (although mineral spirits shouldn't hurt it).
After a few hours (or many), pull the action and hit it with some compressed air while cycling it. When you're satisfied that it's clean enough, hit it with some form of spray lubricant and you're ready to go. Crude but effective (both the gun and the cleaning method). I bought mine out of nostalgia (grew up with old Winchesters) and I liked the weight (16s were built on 20-frames). It won't replace any of my doubles, but it was a fun project and lots of bang for the buck (no pun intended).
