Everything I have read or heard on automotive shows have said synthetic is superior to conventional motor oil. It's viscosity maintains better at high engine temperatures and in cold areas it doesn't thicken up for better start up.In addition there is less friction using synthetic.But like all oils it becomes dirty and contaminated, and I would not leave it in for extended change intervals over 6 months.The old rule of thumb with conventional oil was 3 months or 3000 miles.Although not environmentally sound frequent oil changes are the best insurance for long engine life.