Originally Posted By: Bookmaker
It's not what's best for the car that drives 5w20. It's what's best for the manufacturer in meeting CAFE regulations.
I don't think that logically follows from the document. The document merely states what must be done in order to use 5w-20 oil in the EPA FE tests. There's nothing forcing any manufacturer to actually use 5w-20 oil in applications where it's not sufficient for engine protection.
Take Honda specifically. I highly doubt that if Honda was spec'ing 5w-30 for all the vehicles it specs 5w-20 for, it would fail to meet CAFE requirements. (Maybe I'm wrong, I don't know for sure). Thus, Honda probably isn't being "forced" into recommending only 5w-20 oil.
Further, both Honda and Ford continue to spec 5w-30 in some vehicles (Civic SI and Mustang V6, for instance). So presumably engineering decisions are still impacting the decision of what oil grade to recommend for a given application.
The only thing that document provides is a guarantee that manufacturers cannot spec a 5w-30 oil for actual field use but use a 5w-20 in testing despite not being robust enough.