There has been a 40+ year effort towards using low viscosity motor oil to incrementally reduce fuel consumption and thus improve fleet averages for Corporate Average Fuel Economy (CAFE). In some cases, no changes were allegedly made to certain engines and there were claims that, for example, some Honda engines with a manufacturer preferred grade of, say, 5w-20 in the U.S. would list 10w-30 (or something) in parts of Northern Europe, leading to assumption heavier oil was what it really needed.
However , only someone who was a Honda engineer or mechanic experienced with engines on both continents would really know for sure whether the engines were the same or different. . But, when the Ford 3.0 "Vulcan" was introduced for the '86 Taurus, I recall something about the oil pump being designed to specifically handle 5w-30, a shockingly "thin" oil for year-round use at the time. Ten years later or so, the same thing with the Triton V-10 for 5w-20 and I think I recall Technical Service Bulletins (TSB)s admonishing dealers to not fill them with heavier weight oil both for cold start protection and because there would actually be a noticable difference in fuel consumption, unlike many engines where differences were in tenths of a mile per gallon. As far as full synthetic vs. partial synthetic, the same as weights of oil, if in doubt, check the manual, especially if you want to keep the warranty in effect. Another issue which may invite controversy is the use of synthetics too soon if in an engine that didn't specify it in the first place. There were claims of improper break-in, with oil consumption remaining high for some time. Some claim the Triton (and some others) are to be run somewhat hard; not abused, but not babied, either. And that's with whatever factory fill of whatever they put in it for whatever time or miles were specified.
Someone with an owner's manual might comment on what it says regarding oil, break-in, when one can start towing, etc.
