I have an 2001 Tundra. The dealer's recommendation is right out of the manual. The GTX 5w30 (or 10w30) will work fine, probably even at 5k miles in this engine if you do mainly highway driving and are fairly easy on the vehicle. I would not be able to run it that long though. I don't do much highway driving and would worry about it. I changed mine at 3-4k intervals for the first 20k miles. I am currently using Schaeffer's blend in mine (great oil BTW), but Mobil 1 has also shown to do an excellent job in both the 5w and 10w30 flavors for >5k intervals. Check out the used oil analysis section.
I would definately not switch to a synthetic until 10-20k miles though. You will hear differing opinions, but there are too many stories about high consumption with Mobil 1, and I have not been able to rule out this critical break-in period as being the cause. Until I tried the Schaeffer's, all I ever used was dino. I have never had a car burn a detectible amount of oil within the oil change interval. Of course, I haven't put over 130k miles on any of my cars either. YMMV