First off, if you change your oil every 3k miles then just stick to a name brand conventional oil. I usually purchase whatever is on sale. Lately I've been using Mobil's Super 5000 5w-30, though I change it every 3k. It's almost as cheap as Wal-Mart's Super Tech.
As for those of you who leave your oil in for 6-15k mi., just remember that even though a full synthetic oil can theoretically maintain viscosity for that sort of mileage, your oil filter is still rated for 3k miles. Once your filter becomes plugged a bypass valve will open allowing un-filtered oil to circulate through your engine. If your oil is full synthetic and rated to last that many miles, then please change your filter every 3k and add a quart or so to make up for its displacement.
