There is a huge difference between changing the oil at 6,000 miles and 20,000 miles. Still, I do understand, and appreciate, your point about what Toyota might have to say about it.
The new Mobile 1 Extended oil (which is for 15,000 miles) seems to be a great oil. Still, I change my oil every 5,000 miles. I have been doing so on all of my vehicles (with regular Mobil 1 - which is still WAY better than conventional oil) and never had a problem.
Interestingly, my BMW 545i calls for oil changes ever 15,000 miles. That's way to much for me, even though its recommended. I changed it at about halfway, 7,500 miles, and will continue to do so. BMW will change the oil for free during the first 50,000 miles (but only every 15,000 miles, so any oil changes in between I have to pay for- which cost about $95.)