New info: I removed all the spark plugs again today and see I missed
an important difference among them: Three of the plugs' ceramic areas
(firing end, directly adjacent to where the spark occurs, not the wire
end) are dark and look a bit oil fouled. One looks white-ish, the way
it is supposed to according to photos on the web that discuss spark
plug inspection. Considering the good fuel mileage, I am now leaning
towards the problem not being the breather system but instead oil
control rings on three of the pistons and Jim's and Priyantha's
theory.

I cannot get a good photo of my plugs with my inexpensive camera.