If im not mistaken (someone correct me if im wrong please) dino oil with time builds up deposits on your old, worn down gaskets, seals, etc. Any of them that might be about to break may still be held together with these already formed deposits.
Now, full synthetic motor oil has more detergents. Meaning a cleaner motor. Clean out the old deposits, and the seals will break causing leaks. I recommend a full engine flush, inspect for any leaks, fix the leaks (new gaskets/seals), and use full synthetic. thats what im going to be doing anyways.