I would first consider what environment / climate the car is operating in at the moment. 10W30 may be fine for Canada in winter, but not the Bahamas in summer!
In general. I would have thought that the 308 pre-dates the technology of 10W30-grade oils. I would agree that 20W40-50 (or maybe 15W-40) is better suited to the parameters the engine designers worked to. I'd check the manual & a Ferrari service department too.