LOL. The title of the thread is Not an Oil Thread, but clearly it is one.That is why Honda specifies 10W30 in the NCX engine. Whether the NCX engine had been developed yet or not is immaterial.
Oil temp is an easily-measured manifestation of the additional power and fuel wasted by higher-viscosity oils. Thank you for bringing that into the discussion, as it is a much easier way to see the wasted fuel that drove Honda (who have had a core organizational philosophy of using less fuel and other resources for a long time) to make the viscosity recommendation change.
Interestingly, if you watch oil temps what you'll find is that there's an optimal* fluid viscosity. Use a too-thick fluid (vs optimal) and oil temps rise. Use a too-thin fluid and oil temps also rise. The former is due to wasted power and fuel. The latter is due to excess metal-to-metal contact and wear. This reality is how we know that the oil temp work you refer to was about wasted fuel. Had the data they collected been used to INCREASE viscosity, that would have told us the recommendation change was about wear and durability. That the outcome was to REDUCE viscosity demonstrates that the recommendation change was entirely about fuel savings.
*NOTE: "Optimal" depends a very great deal on use case. Racetrack or high-speed Autobahn use of a small-displacement engine has a very different 'optimal' point than average American-street use of a larger-displacement engine.