I've always thought the conventional wisdom was you should change it every 3000 miles, the only exception being vehicles using synthetic oil, which can go a lot longer between changes. And if you want to run synthetic oil, you need to do it basically for the life of the engine; switching to it once the engine has some miles on it (supposedly) isn't really a good thing to do. This isn't expert advice, I'm just repeating what I've been told.