IMO, mfg spec thinner viscosity to satisfy EPA for mpg requirements.
I don't think it is intended to have a longer engine life.
I recall reading this in some of the industry trade mags when I was working. In addition, those that use oil for things other than lubrication, such as VVT cam phaser actuation, are concerned with a minimum oil flow across the ambient temperature range for North America. And minimal desposits.
This is my only hang up when it comes to switching to a heavier oil, VVT cam phasers. I don't know enough about them. Do they need a "thin" oil to function properly?
Down in the 3rd party additive forum, a guy said that he put a bottle of Restore (thick) in his Acura and right away the MIL came on with VVT codes. He drained the oil and refilled without the Restore and all was well.
My 2012 Fusion with 2.5L has always made some valve train noise. Now I live in NC. In 2019 we had 89 days that were 90 degrees or hotter. I thought about switching it to 5W30, but I am concerned about the VVT.
On the other hand, I have always run MC SB 5W20. There is about 115k miles on it now. Last change I went 8k miles (20% OLM) and had a Blackstone UOA done. Everything was excellent, no issues. So, maybe I don't have anything to worry about?
My daughters 2013 Corolla 1.8L has 196k miles on it. I run Valvoline FSHM 0W20 in it. It burns about a quart and a half in 5k miles. So, I thought about switching it to 5W30 to see if that would slow down the consumption. Again, I was worried about the VVT though.
One other thought...my 06 Tacoma 2.7L calls for 5W30 and it has VVT. Is it the "same" system as the Corolla, or is the design somehow different to call for heavier oil?
So many questions, so few answers...