There is NO problem switching from Dino to Synth oil or Vice Versa.....
Also consider that in newer oils the zinc additive ZDDP has been reduced significantly to protect emission equipment in cars. This can have a detrimental effect on older / air cooled engines. I believe some of the "High Mileage" versions of oils come with higher amounts of ZDDP. You can also buy the additive from Lucas, GM and others.
Another factor to consider is break down at high heat and Flash Point. Many dino oils do better than synthetics in this category.
That said, I've used synthetics since the 80's and have never had any issues. IMO, their pluses outweigh their shortcomings.
PS: Many (if not most) "Synthetic" oils sold here in the US would not be considered synthetic in Europe. It's actually pretty tough to determine what is a "true" synth. Unless you know for sure what basestock is used and how it's blended you'll never know for sure.