The Engineers don't really spec the oil, if they did, you'd arguably have a range of viscosities in the manual, like things used to be. I know I've brought it up many times, but we have several examples of where the oil recommendation changes significantly depending on the perceived usage profile. The Mustang GT for example, back when the Track Pack was available, the "regular" GT spec'd 5W-20, the "Track Pack" version with the same engine, but a less conservative thermal castration mechanism, and oil cooler, spec'd 5W-50.
Of course the "regular" mass produced HEMI spec's 5W-20, while its HD and high performance siblings spec 0W-40, despite sharing myriad components. Clearly, usage profile weighs in there.
Engineering recommendations are kneecapped by CAFE regulations which mandate a single spec oil grade.