changing the oil at 3K (full syn) is a little pointless.
I used to do this, and I noticed the oil was the same color going in as it was coming out.
I see it this way:
3K: Jiffy Lube wants you to frequently change it so they make more money
7K: Toyota wants you to run your engine slowly into the ground so they can sell you a new car or new engine after 100K
5K: HAppy medium between the two. I change mine roughly ever 5k-7k depending on the type of driving and weather. It's been 6500K since my last change, and the oil is STILL amber. I'll probably change it in the next few weeks.
My dad runs Mobil 1 (like me) and changes it every 10-15K and doesn't seem to have any problems. Then again, he doesn't rev to redline all the time like I do