i read somewhere that conventional oil starts breaking down almost immediately whereas synthetic starts breaking down at only 3k or so (i think)... meaning, there is no harm in changing synthetic at 5k, at all. especially if you're driving more highway than not.

save your money and do 5k. you'll be safe. the ones saying 3k are just repeating what they've heard.. i'm willing to bet they haven't done any research to back up their claim.