Dead Dino's will leave a film behind and coat the engine.
If you have been using synthetic long enough there is no more film or dirt left in your engine to cause any problems at this point. I would continue to use it.
I would recommend that if you want to use synthetic you start early in the engines life to avoid this type of problem.
Same it true for automatic transmissions, don't switch unless the trans is not that old or you are asking for problems.
Been there and done that.
