Dale,

My thought would be worn out valve guides more than defective valve stem seals. The stock 235 had very mediocre seals to begin with. I think this is what your friend was alluding to.

It could also be a bad ring or rings that don't show up when roaring down the freeway as the smoke is dispersed. Your compression values are great, but a score down a cylinder can allow oil into the combustion chamber.

A top end rebuild with knurled guides or new guides would probably solve the problem, but be aware that sometimes a new top end will exacerbate any wear in the ring area and you might end up with a real smoke bomb. This doesn't always happen though so It is probably worth the effort.

Stuart