Posting this in case it helps someone in the same boat.
Bike is CB500K1. The oil pressure sensor quit working (wouldn't light the bulb with engine cold and key on). From reading on this site I figured it had crud inside that had jammed switch open (bike is just running again after sitting 20-plus years). Called my local Honda dealer for the replacement part 37240-P13-013 (replaces 35500-333-014) and was quoted $46. Before shopping for a cheaper aftermarket option, I thought I'd try to clean the old one before I tossed it. Although it's a sealed unit, I soaked in Simple Green to start, then near-boiling water for a couple of minutes, then topped off the cocktail with some WD-40 (the red straw fits exactly in the hole), then blew it out with a bit of air. After this I put a small screwdriver inside to gently push and I could feel it move approx 1/8 - 1/4 inch, which I presume is what it does when oil pressure pushes it.
So it's back on the bike and working as it should (light goes on with engine cold, then goes out after it starts). Who knows how often this would work or how long it will last, but maybe worth trying before replacing a "bad" sensor.
