I have never run a synthetic oil in any of my old school engines, I do use VR-1 but 10w-30 conventional.
The oil weight is determined by the bearing clearances when the engine was built, opinions vary on this topic but typically you do not run 50 weight oil unless you have some pretty loose tolerance's.
It would be interesting to see if the port side of the valves are caked with oil.
If they are not the oil is most likely coming from the rings. It's gett'n in every cylinder.
Have you done a compression test or leak down test lately?
Do you know what valve seals you have now?
