Hopefully the Guru's will jump in here. I believe it's not a matter of your lube lasting longer, it's a matter of it getting contaminated with carbon, un-burned gasoline and condensation from heating and cooling. The syn's will proboly last more than 5,000 miles and not suffer viscosity breakdown or loss of lubricating properties, but it gets so contaminated you need to change it anyhow. I change mine every 5k.5k for me with mobile 1 20w50.
3,000 to 3,500 - Just like the oil in my wife's truck!