Charles T. Low wrote:

So far, I replaced the spark plugs and since then have had no trouble. That
doesn't mean the trouble is over, but preliminary indications are good. The
electrodes or whatever they're called were black, and there was oil on the
screw threads, moreso the further aft on the engine (which sits a bit lower
than the front) ... what this means I have no idea and so far no one else
I've asked does either.

And this is an interesting point. Taken together with your
consumption, it could be sucking oil from the guides on start/warmup
that's sat in the head (proportionately moreso aft), and eventually
fouling the plugs. Have you checked for any smoking at startup? You
could also pull your new aftmost plug after a brief startup/shutdown to
see if it's likewise getting oily. If it were the case the problem
would resurface after awhile & let you know.

Nice book, BTW.