In case anyone missed it, here's a recent post I wrote about why I think it should work.
http://www.toyota-4runner.org/4th-g...rebate-mobil-1-synthetic-oil.html#post1957608
Basically 0w20 is in the same range as 5w20 at operating temperature, but it's better at startup. The TSB approves 5w20 for our engines, so 0w20 should be better. I haven't found any good answer for why they didn't approve 0w20, but it could be that Toyota's own 0w20 is at the low end of the viscosity range to maximize fuel economy (Mobil 1 is a bit higher, I think), or because of cost (6.5 quarts of Toyota 0w20 is expensive), or because it wasn't worth the time/effort/cost to test it, and they won't approve it without testing it. It could be that they only tested one grade lighter than originally spec'd for each engine. Only Toyota knows.
I think it's true that as the engine wears a higher viscosity oil is more appropriate, but from what I've read the 4.7L is easy on oil and the engine will last forever, so it could be a long time before it needs higher viscosity. I don't know as much about the 4.0L. And I don't have a very good understanding of how oil lubricates an engine, so I can't think intuitively about what's going on.
Re: 10,000 mile oil changes. Oil, engines, and filters have gotten better over the years, so there's nothing wrong with a 10,000 mile oil change if that's what it's designed for. 2,500 miles is excessive, and you are most likely draining out clean oil. You can get a used oil analysis before you change it and they will probably tell you that you can go a while longer. Mobil 1 will even give you a warranty against oil-caused engine failures with up to 10,000 mile oil changes (10,000 miles or the manufacturer's recommended interval, whichever is
longer) if you document all the oil changes and if they determine that the failure is caused by oil. The chances of engine failure and them finding that it was caused by oil are low enough that it's not worth it to me to keep documentation, but it might be worth it if you are concerned. Note that their warranty is only good if you use the oil the manufacturer recommends, but their 0w20 says it's approved for any application that calls for 5w20, so you might want to call them and clarify (or better yet, get it in writing if you actually plan to use the warranty).
There was one report somewhere of someone using 0w20 in a 4.7L V8, but I can no longer find it. There are a couple reports of dealers accidentally putting it in, but they are not helpful.
Some UOA reports:
Blackstone UOA (2UZ-FE)
May have went a little too far btw/ oil changes | IH8MUD Forum
http://www.toyota-4runner.org/4th-gen-t4rs/165006-blackstone-oil-analysis-report-2uz-fe-v8.html
Oil Analysis 2UZ-FE | Page 2 | IH8MUD Forum
Lots more if you search.
I'll try it, run it for about 5,000 miles if there are no bad noises, then get a UOA and post the results (probably in the fall).