As near as I can tell, the only benefit to changing it more frequently is it gives the do it yourselfers something to do that they can do.
There have been multiple samples sent out at varying mileages and intervals and every one of them has been fine.
How long do you want the engine to last? Eventually engine components wear out and fatigue regardless of oil change frequency.
Lastly to undertake a study like you said in your post, in order for it to be unbiased and correct, would cost several hundred thousand dollars and run for multiple years. As a matter of fact the seventh gen 4Runner would be out by the time the 5th gen engine oil life study was completed lol. Pretty sure Toyota knows the frequency required hence them warranty-ing with 10K changes.
We have oil systems on large compressors at my refinery, 7000 to 8000 gallon reservoirs, that have the same oil in for years, I mean YEARS!!! We have a very rigid oil analysis program, PM the filters and system on a regular basis, monthly. In fact one system I know has had the same base oil fill for more than 12 years. Now oil is added as it is consumed, leaks, etc...but I can assure you the service is far, far greater than a 4.0L V6 engine...at $15 a gallon, fill and drains can be expensive.
Lastly, I really feel Mobil1 synthetic can go to 15K without any issue, my opinion based one what I have seen in the lab and testing. Me, I will change at 5K, every 5K, because like 1Engineer said, I like doing stuff like this.
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