I've recently moved house and inherited a wood-burning stove. The
doorknob is unusably stiff. I've put on some WD40 equivalent (as
advised) and worked it back and forth. It seemed to get worse the more I
worked it. I then tried engine oil - it also got worse the more I worked
it. In neither case did I light the fire. It's now practically seized.
This behaviour defies common sense (well, mine anyway).
The knob consists of a threaded rod which goes through the door, one end
of which has some rod welded at right angles, the other end has the
handle screwed onto it. The thread seems in good condition.