I'm not a tech but, here's my cut at your question...
There is no issue with synthetic oil per se. It is better than dinosaur oil in almost every respect (except cost). The issue is switching a car over to synthetic after it has used dino oil all of its life.
Over time, dino oil tends to build up deposits in the cracks and crevices inside the engine. These deposits act like "false seals". When you switch to synthetic these false seals are removed which often causes oil leaks at the gaskets. This creates a costly problem that would have never occurred if the car remained on dino oil.
When Benzmac rebuilt his 190E 2.3 16V he switched to Mobil 1 and said that was all he would ever use in it. He would not have switched if he had not just replaced every gasket in the engine.
Bill Wood - Webmaster