There’s really no way to know what actually went into the decision. I know from experience that the tolerances that engines are manuafctured to are far, far tighter than they every were in the past due to a concept called “statistical process control” that manages the processes by controlling variation rather than controlling the parts to within tolerance ranges, and I know that these much tighter engines may need lower base-weight oils to break in properly, but whether that or CAFE requirements drove the engineering decision is tough to guess. I can also tell you that countless hours of reliability testing, qualification testing, and subsequent engineering went into the decision, but have no way of knowing how much it impacted it.
I seriously doubt the planned obsolescence theory.
The bottom line is that I believe it’s better to simply follow the prescription of the engineers. No good comes from second guessing them. Unless, of course, you is one. (just kidding).