Buy the oil at WM. Mobil 1 is a lot cheaper than Toyota oil.
I just bought some Toyota 0W-20 synthetic at the dealer for my Toyota Land Cruiser with the 5.7L engine and it was $4.95 per quart.
I highly recommend this Toyota oil to the OP for his Tundra
Sure, Mobil 1 0W-20 AFE is approx. $23 per 5 quart jug (about $4.60/quart) at Wal-Mart so it is a little cheaper, but not "a lot cheaper than Toyota Oil."
For my money, the Toyota Genuine Motor Oil (TGMO) 0W-20 is a much better oil than Mobil 1. No doubt, either one will get the job done, but for just a little bit more it seems to make a lot of sense to stick with the Toyota oil.
To the OP:
Part# for the 0W-20 Toyota oil is 00279-0WQTE-01 when purchased in one quart bottles.
You might also want to consider the Toyota TRD Performance oil filter in lieu of the standard Toyota oil filter.
Standard filter on the left, TRD Performance filter on the right:
The standard filter is a cellulose filter while the TRD filter is a 100% synthetic glass/polyester filter that's 98% efficient at 20 microns.