and to add to the majority already posted, if you're running synth right now with no leaks, you MAY be okay with Mobil 1. Yes, synth oil will remove deposits of dyno crud left back from years gone by. And yes, dyno oil allows rubber seals in your engine to expand over years, while synth will instantly return them back to their original size. This is the largest problem you'll have with leaks! Once a seal has expanded, if you reduce it's size with synth oil, you'll have leaks at each one of them seals.
Since you're using synth blend right now and don't have any leaks, you'll probably be okay. The good news is that if you DO get a leak after switching, go directly to a dyno oil and run that for a few weeks, your seals should swell right back up. Bad news is that you may not be able to go back to a synth blend as your seals may not take well to any more synth...
As for the only 100% synth, there's a ton of brands out there. Redline has a full synth, so does Castrol, Quaker State, and I think Penzoil even has one now. I personally use Royal Purple in my truck and Durango, and both run very well with the grape juice! I used to buy Amsoil, but have found that they are just overpriced, I can get just as good oil-if not better- for a few bucks less....
Just me rambling!
Chris