At the end Taylor mentions we would have to ensure the oil had adequate durability. The paper was about reducing friction and the reduced fuel consumption that comes from that, resuting in lower CO2 emissions, and was not about durability.
So what is " adequate durability " ? From various sources on the internet, fewer than 1.2 % of vehicles achieve 200,000 miles, for all sorts of reasons, engine wear out being just one of them. Pretty clear to see that making an engine last 200,000 miles would be " adequate durability" for an auto manufacturer. Actually, that number could even be lower. Would auto manufacturers care if 5% of their engines wore out and became knocking, oil burning heaps prior to 200,000 miles?
From postings I've read, most bitgoers want more than 200,000 miles out of their engines, although I wonder how many sell their vehicle before then. I would like to see 300,000 miles on mine, and I'm closing in on 200,000 miles in a year or two. This is on a Suburban I keep for towing. To replace it might cost $70,000 to replace with a new one. I've already replaced the front 4 wheel drive hubs with that objective in mind.
So, as far as getting that 2 to 4% reduction on gas consumption and reduced CO2 emmisions, you can count me out. However, I'm not convinced just going to a xW40 is the right answer for everyone. ( but I am convinced enough to run 5w40 when towing in the summer and the oil cooler is being taxed in the 90 degree weather. ) What I'd like to see is more empirical data on engines that achieved 300,000 miles on their respective viscosity. Also, I need to see this in sophistcated engines with turbos or cylinder deactivation, not grampa's old V-8.
An example is the re-design of Chevy's Gen 5 V-8 in 2014. The 2018 owners manual calls for 0W20 in the Gen V, but still asks for 5w30 in the 6 liter V-8 which is still a Gen 4. It actually says in the manual not to run 30 and 40 weight oils in the 5.3 and 6.2 Gen 5 engines. If someone is running 5w40 in a Gen 5 V-8 since the warranty came off, and they have, 300,000 miles on it without problems, that would be useful. Trouble is, we will have to wait years for that kind of info.
Meanwhile, excellent paper on better gas mileage and reduced CO2 emmisions. Thanks for the post.