It could be a worn guide/valve stem seal or rings. Since it doesn't smoke under a heavy load, I'd guess guide/seal.:yes:
There's a coouple other things that COULD cause it. :yes:
PVC (or bad vacuum modulator on a th350) sucking oil in (depending on the location of the vacuum port) and 99% of the oil going into a couple siamesed cylinders. This is mostly common with dual plane intakes since only 2 cylinders on each side actually get their air/fuel mix from that side of the intake.
As for the noise, it could be loose lash or a header gasket leaking, which will also help destroy the plug boots and cause a miss.
It's easy enough to do a compression test and leak-down to make sure the bottom end is sound. The rest of the story will be realized when the heads come off.:yes: unless you have a lighted bore scope to LOOK in through the plug holes..:devil:
Happy hunting.