Hokiefyd/Jason, you're correct Jason, the NHTSA doesn't require the Honda Motor Company ("Honda") to use a particular weight motor oil. I didn't mean to imply that. That's what comes of trying to nutshell the concept of a federal sales-weighted harmonic mean fuel-economy standard for a manufacturer's fleet of current model-year cars. (Whew!)
As you know, there are specific sales-weighted fuel-economy categories, and to extend that a manufacturer is able to achieve a mpg number that is better than that prescribed for a particular CAFE category, the manufacturer can earn fuel-economy "credits" which can then be applied to other sales-weighted categories in the manfacturer's fleet or sold. Honda sells its credits annually, and makes a great deal of money doing so. (I'd have to check, but for the last year that figures exist -- 2005? -- Honda earned something on the order of $250-280M through the sale of energy credits to BMW alone.)
And, as you stated, manufacturers are required to recommend to their customers the same lubricants that they used during the certification process. The point that I was trying to make is that there is no ambiguity about what Honda is "recommending." In this regard, under the category of "Recommended Engine Oil," Honda's current owners' manuals state, in part, "Always use a premium-grade 0W-20 detergent oil displaying the API Certification Seal." That's what I was referring to when I wrote that Honda's recommendation is really a mandate, and that Honda specifies that specific oil weight in order to comply with the company's previously having gained certification of a sales-weighted category of cars using that weight oil. Hard to nutshell that. You did it much better.
All that said, I believe one can go to school on the fact that Honda, clearly obsessed with fuel-economy, still recommends 5W-30 weight oil for its higher performance engines (10-30 in the case of the S2k), and 5W-30 weight synthetic oil for the turbo-charged engine in the RDX. The only logical conclusion is that 5W-30 weight oils, conventional and synthetic, must offer additional protection over 0W-20 weight motor oil in a HT/HS environment.