Synthetic Motor Oil can be used during break-in trouble free. In fact, vehicles such as Dodge Viper and Chevrolet Corvette come factory filled with synthetic oil.
Since most new vehicles come filled with petroleum oil, it only makes good sense to change to synthetic at the first scheduled oil change interval. New engine components generate high levels of wear metals and can contain contaminants from assembly. By allowing the engine to operate with the petroleum oil until the first oil/filter change interval, the wear metals and contaminants are removed prior to installing synthetic oil. If its not useing oil then the rings has seated. whether the rings are seated or not has nothing to do with the oil pressure.