Below is the trigger group for my K-31 - I marvel at its simplicity and functionality.
Here is a shot of the bolt disassembled. It is also a wonderful piece of engineering. It wasn't gunked up too bad and was pretty easy to disassemble and reassemble.
After reassembling, I greased the parts of the bolt that required it. Yes, grease, not oil. The Swiss always used grease and I purchased a lifetime supply from a gentleman in Switzerland along with a couple of other things. It is almost a liquid (but not quite) at room temperature. It is called Waffenfett. Later on the Swiss used Automatenfett, which is more of a black graphite grease and I have a tub of that as well. Below is the yellow waffenfett.