Mobil 1 > Royal Purple. Lower friction, better detergent. It's really great if you can find the 5 quart bottles on sale at Wal-Mart (which happens from time to time).
BTW, the Amsoil 0W-30 I'm running now is API certified and recommended drain interval is 1 year or 35,000 miles, whichever comes first. Kinda blows the "every 3000 miles" bit out of the water, doesn't it?