I read that page Mike sent you and I interpret it as saying that your oil must meet the SAE and API grades and specs. I also see it as a warning to folks to maintain the change interval required. That was a big problem when synthetics first hit the market.
The obvious advantages that synthetic oils provide does not include longevity. But, people thought that, since they were paying four times more for the oil, they could extend the change interval four times too. That caused a whole lot of "miscommunication" between engine manufacturers and customers when the engines failed.
I see you don't have that problem. It is apparent that you already know to maintain the change interval just as you would with petroleum based oil.
One other problem with synthetics has been oil seal material formulation. Some seals are made from materials that degrade much faster when exposed to synthetics. I don't think that is a problem with the Honda crankshaft seals but I don't know for sure.
So, like Mike said, be careful about your warranty. Cross your I's and dot your "T"'s ( or something like that). Make doggone sure that you keep records and that your practices will hold up under scrutiny if anything goes wrong and you have a claim. In these economic times, even Honda will be selling fewer products and will be watching every penny.
happy boating,
jimmyd
