I had a similar problem when I switched to Syntec 5w50. I had made the change
to get rid of the lifter noise, it worked. But I did develop a leak, the
engine had only 10,000 mi. on it at the time so I figured that it wasn't
dried out seals. My leak turned out to be from an imperfection in the case
where the oil pump fit. My guess is that for some reason the synthetic oil is
able to escape through smaller tolerances than dino oil. My leak didn't go
away after a couple of thousand miles so I figured it had to be mechanical.
Probably the easiest thing to do would be to switch back to dino and see if
the leak clears up.

Good Luck
Derek
'87 Syncro