[aporigine]

I would not go to a different viscosity. A 5W oil is plenty thin enough for winter starts in Dixie, so a 0W oil is not needed imo. It's the hot-viscosity number after the W that is of more interest. 8th-gen Honda engines are spec'd for 5W20 oils, but if i remember correctly, you like to operate at low rpm and high torque as befits a hypermiler. I would not go lighter than a W30 oil. In fact, there is some controversy (I don't know how solidly founded) about the adequacy of 20-weight in the later Honda engines being driven for performance, i.e. at higher revs where there is no question of the oil pressure being ample. As I haven't heard of any 25-weight oils...
My recommendation would be to go with a full-synthetic oil in 5W30 weight. The best synthetics, like Redline, Amsoil, Motul, have a lower viscosity than the corresponding regular oils while retaining film thickness and strength. This is the basis for their improved fuel economy in use. For stuff you can get at mall-wart, my pick would be Mobil 1. Mobil has been branding some semisynthetics, so make sure you're getting the full-ticket premium synthetic. The regular and semisynthetic oils tend to use hydrocracked petroleum stocks as the base. Not bad, but the viscosity index (how resistant the oil is to thinning with heat) is rather better with a full-synthetic PAO base stock. (poly-alpha-olefin, made from scratch and not by refining and modding a crude oil cut) The consequence of this is that the oil can meet 5W30 specs with a smaller proportion of viscosity modifiers (than in an equivalent-weight oil using mineral base stock), gel-like molecules that thicken hot oil but "fold up and disappear" in the cold. Because these molecules are long, they degrade under shear stress faster than the base stock, be it mineral, hydrocracked or full synthetic. Less viscosity modifier = good! (imo) I grant you that full synthetics are expensive - expect to pay $20 above regular motor oil for a 4-qt fill. But if you gain 0.5mpg over say 7000 miles of the oil's lifetime ... at these gas prices you've come out ahead. (smile)
cheers apo