Here is a post I have written in the past to explain a little better:
A 4Runner will do many things but it will not travel in time. This is not 1972 and what your dad and granddad remember is not applicable today. I do not know how much you understand about synthetic fluids except that in reading your post I can assume not much. How can you be "shocked" to see Toyota putting a 0-20 Full synthetic fluid in Houston? I will try to explain.
I am citing Mobil 1 full synthetic specs as this is an average in the industry.
10W-30 Mobil 1
VI=146
Cst at 100c=10.1
Cst at 40c=63.2
0W-20 Mobil 1
VI=173
Cst at 100c=8.7
Cst at 40c=44.8
For your edification "VI" = "Viscosity Index" and "Cst" = "centistokes" which is a measurement of it's resistance to pour, or "thickness for purposes of this board. Most on here are not "Bobistheoilguy".
The higher the viscosity index the better. It means that the oil will maintain a usable viscosity over a greater range of temperatures.
As you can see the 0-20 Synthetic has a smaller delta (delta means rate of change) from 100C (212F) to 40C (100F) than the 10-30. This means that the 0-20 Fluid has a "flatter" curve throughout than the 10-30, which means that 0-20 will maintain a better viscosity throughout this temperature range.
What I explained above is really a moot point though. Either a 0W-20 or a 10W30 full synthetic will offer outstanding protection for your Houston summers. You have to remember these are full synthetics - they are different. Your engine won't be able to tell the difference under normal operating conditions.
In closing, the 0W-20 synthetic is a good all purpose choice no matter where you are in North America. The differences are small in the temperature ranges our engines operate - including Houston.
Now, break in shavings? C'mon. Toyota offers a 10 year bazillion mile warranty for under a thousand bucks on these vehicles - and they recommend their FIRST oil change to be at 10K under normal driving conditions. If you travel offroad a lot or pull a heavy trailer it is 5K. NOT 1K to get rid of "break in shavings". Do you really think they are going to offer this warranty with this maintenance schedule if there are shavings in the motor at 1K miles? No, I don't think so either. Trust the manual. It was written by some smart guys.
And a link to a thread that should answer all questions. This is for the new guys
http://www.toyota-4runner.org/5th-gen-t4rs/109588-oil-change-0w20-wtf.html enjoy!