I wonder if the higher end synthetics have better detergents which may have cleaned up some sludge that may have prevented some oil from seeping through the seals? If so, it could explain why you're now seeing symptoms of the infamous valve guide seal issues.

It could also simply be coincidental failure of the seals around the time you decided to use synthetic oil.

In either case, if it were me, I'd simply look to change the seals as soon as I could and continue using the best possible oil I could afford. <img src="/forums/images/graemlins/kewl.gif" alt="" />