If I'm not mistaken, I've noticed on PDS form other oils (M1 comes to mind), that say to use this grade when/where recommended by the manufacturer (honda, ford...). I don't find this odd, but perhaps I don't see it in the same conceptual context.
As far as noting such restriction towards use, I would think the reasoning falls under liability. They're just protecting themselves should someone throw this oil in say a high mileage towing vehicle with higher clearences in the heat of summer. Things just may come to a head over this OCI where the engine gives and since this is one of those times where the person wanted to try something new and perhaps hoping to gain some mpg...they may look at the oil to blame, maybe not.
Unless bearing surface area is spec'ed for such a lighter weight oil and the clearences aren't excessive, I believe the wear rate study based on lube HTHS may play out with increasing wear rates. I do also understand that additives play a role to, but only so much I feel.
This is nothing more than my opinion.
