Well, finally got to this job. I had known it was necessary quite a while ago, but it took me quite a bit to get all the pieces together and to feel prepared to go. So, after removing some stuff to get at it, here's the valve cover off the passenger side:
Here's a close up of the same thing, and you can see those black rubber gaskets around the tubes that the spark plugs go in have built up crud. They were what was leaking. I think they just get cooked over time and don't work well.
Here's the inside of the cover, after wiping away the wet oil. I would guess those burnt marks are where oil has been cooked onto the aluminium.
Here it is after I cleaned it up with some solvent:
Then, as I went in to change the spark plugs, I discovered that the end of the wire from the distributor -that connects to the plug - had broken off and came out on it's own, rather than at the end of the wire. Not good. Had to go to town and buy a set of 4 wires. (they don't just sell one)
But, I got everything back together, did a bunch of cleaning, and it seems the car is running well. Here's the engine, looking clean, all back together.