If you want to dig into the science of this, the definitive website is BITOG:marcwd wrote:
What about the need for contaminants to be flushed from the lubrication system on a regular interval regardless of the type of oil used? Maybe this is Honda's position.
There is a lot of technical reading there about how oil is made, the difference between conventional and synthetic, why it needs to be changed, and a lively discussion forum. The popular thing over there is users get their used oil analyzed by a lab to monitor engine wear and figure out ideal change intervals.
If you read though threads and look at the lab analysis results and commentary usually oil gets to a point where it needs to be changed because the additives get used up (anti-wear chemicals, detergents, and in the case of conventional oil friction modifiers) before the point that it builds up too much contaminants... Unless of course the engine has a major mechanical issue that needs fixing - in which case frequent changes wont help.
I think the bigger issue with Honda's guideline (and its in every manual I have ever read also) is that there is so much variation in what constitutes a "synthetic" that it would be very hard for them to come up with a recommendation that fit all oils. They would need a rule for MobilOne, a rule for Pennzoil Platium, a rule for Castrol Syntec, a rule for Amsoil, and so on...
Much easier and lower liability to just have one basic recommendation for everything.