[zebra1550]

You won't screw up your warranty at all by switching to synthetic. Toyota sets guidelines as to the types of oil you can use. I can tell you that Mobil 1 will far exceed they're minimum standards. I use Mobil 1 here in Nebraska where it can get as cold as -20 in the winter and routinely gets up to the 90's in the summer. Synthetics are the only way to go in your engine if you want to make it last. I also have a 2003 Tundra that I switched my oil to synthetic at 1K miles. I'm just getting ready to change the oil at 6k miles and it still looks clean on the dipstick.