ALL of the old rubberized cork or pure cork gaskets which seal oily fluids will eventually start to seep, by observation. Some dino oils have more thinner components in them than others do, which relates to the brand of the motor oil, not specifically that it's dino or syn. Which is why some brands of oils would cause valve cover gasket leaks in engines and others were less prone to leak, by observation. Might have also had something to do with the viscosity of the oil, too?
My late machine shop operative noted that you wanted a motor oil that might cause oil leaks. The thinner factions of the oil were getting places that other oils might not get, he noted. Which is why his old pickup truck would have a valve cover gasket leak with one brand of motor oil and when he changed to another brand, that leak would dry up. He bought whatever oil was on sale at the local auto supply, one case at a time, back then.
Once the oil wicking action into and through the cork-material gaskets starts, it might be slowed a bit by tightening the hold-down bolts more, but most probably not. Which is why valve covers almost always have indentions under the hold down bolt heads!
At one time, some claimed that the syn oils would compromise rubber lip seals in the engine. As in timing cover front crankshaft seals. I'm not sure that was universally true, though, although at one time, Valvoline recommended their syn oils for engines built after 1975 or so, which I read on one of their old oil bottles in the 1990s. Many people have changed to syn oil in their older engines with no issues, though. I don't know that I would put it in an older engine with lots of sludge and gunk in it though, but if that engine had been rebuilt, THEN it might be a good decision. Several variables to consider.
Enjoy!
CBODY67
