I don't think it's a headgasket, the car is never down a drop of coolant.
Here's some pics I found, descriptions said the plugs were from engines that burned oil
And here's the pic from the back of the Haynes manual:
The engine goes through a quart every 1,000-1,200 miles so it would make sense. Since it still runs strong I suspect it's valve seals, but changing them means having to reset the preload on the rockers, not sure I am up for trying that myself.