Synthetics will give you wider operating conditions and resist depositing varnish and sludge build up. But it is a myth that one can run longer without accumulating contaminants as that is a function of the engine's condition. Synthetics will resist break down longer.
Wes is correct in that the best oil is fresh oil. There is a great thread in the Q45 Forum archives, or read on the web at sites like
http://www.bobistheoilguy.com