Very interesting read, Jerry. Thanks for the link. I read another interesting SAE paper on the same subject by 4 other Japanese researchers that outlined some of the testing that Honda did on ultra-low viscosity oils. Honda's results differed from these researchers in that Honda found that friction (and wear) increased once HTHS viscosity fell below 2.6cP for conventional engines (ones without roller cam followers). Honda didn't find the bottom limit for roller cam follower engines. These researchers found friction reduction all the way down to 2.1cP but didn't mention what kind of engine they were testing.
This paper had some surprising discoveries about phosphorous--that it lead to higher friction in motor oils. Also, it underlined the problems with additive clash. I'm more cautious about mixing oils now.
I have experimented with Mobil 1 0-20 oil in my Mazda Miata. What I noticed is a loss of oil pressure, and a possible slight loss (0.5%) in fuel economy vs. my normal 12W-40 synthetic oil. ( for what it is worth, I mix 2 qts 10-30 and 2 qts 15-50, for oil pressure reasons).
What I did not see is a reference to an older engine. As my car has 100K miles, it just might have some wear:)
It seems that in my case the thinner oil allowed more blowby and less effective ring sealing.