I would avoid any additives and just continue to run the synthetic. From your description, it sounds as if you may have purchased the car within the last several months, and if the previous history is in question, there may be sludged rings (and other areas). You'll get a hint of that when changing the valve stem seals.
If the rings are sludged and the changes of synthetic are now cleaning away the contamination, that could be the reason for the oil smoke. Once the engine is cleaner and the oil ring rails have a chance to move and expand, oil control might improve. Your leakage test seem to indicate the compression rings are doing their job, and the majority of the oil smoke may be from the valve stems.
The 231 Buick is an honest 300,000 mile engine if reasonably cared for. Keep good clean oil in it and take care of the valve seals, and any additional smoking from oil rings may clear up.