First, congrats on keeping an old and reliable car running!
Originally Posted By: demarpaint
You could try a piston soak, and a Kreen treatment. Follow that up with a high mile oil and hope for the best.
Piston soak....yes. I saw a video on YouTube recently about how some Toyota piston rings didn't have sufficient oil drainage which caused them to become blocked and the engine to start using oil. If you can somehow get those rings de-coked it should stop using oil.
If it's not the valve seals (easy and inexpensive) it's likely the rings (less easy and more expensive). Choices as I see them and the order I'd proceed:
Piston soak.
Valve stems.
Decide whether to add oil frequently or do the re-ring.
Lastly, you might want to try a good cleaning synthetic...that might be enough to fix thing, but would likely cause more leakage initially...
edit: as a matter of fact, using conventional oil for your OCIs might have helped cause the issue in the first place.