This is going to turn into a flame war...I can just feel it!!!
OK, here goes -
Page 11.7 in the service manual lists 5 "symptoms" of unusual "events"
EVERY one, the first cause is likely to be "faulty" engine oil, ie incorrect engine oil.
I wouldn't agree that 'faulty' necessarily means incorrect. How incorrect does it have to be to be classified as faulty?
...There IS a clutch of sorts in the HFT to enable neutral to drive activation.
Well I've not stripped a DN-01 engine (unlike most other Honda engines of 30 years ago when I conducted the technical training on them at Honda UK), but unless I'm very much mistaken, there is NO clutch of any sort whatsoever in a DN-01. The fact it can emulate Neutral does not a clutch make. A clutch is a specific device with 2 sets of plates clamped together and the drive to the rear wheel(s) is via the friction between those 2 sets of plates. It is this exact point that is relevant to the use of synthetic oils in motorcycles which are unusual (compared to cars) in mostly using wet clutches. The friction modifiers used in many modern oils (commonly but not uniquely synthetic) might allow an otherwise healthy clutch to slip (apart from Ducatis with rattly dry clutches of course). This is the reason why some oils are not suitable for motorcycle use and a motorcycle specific oil will simply be formulated with fewer such modifiers to allow wet clutches to work as intended.
The DN-01 has no such clutch and hence the friction modifier content is irrelevant.
Also, regular engine oil is designed to work with SLIDING parts, not cogs and gears. That's why cars have separate engine and transmission oils.
But motorcycles usually run the same oil in engine and gearbox. It can be argued that separate oils specifically targeted at those different uses would be a better solution, however the statistics back up Honda's claim that it makes sod all difference as engines with common oil in main engine and gearbox experience NO deleterious effects of this sharing. Honda cars with typical separate engine and gearbox housings used to use the same 10W40 in both. Same oil, just not actually shared simply due to the physical design of the engine and gearbox, either side of a dry clutch.
The important points about which oil to use are the specs that Honda quote. This is the oil standard (API, JASO etc) and the viscosity (how thick it is). Both are crucial. Honda quote an oil standard that must be met - or exceeded. That last is important since oil technology progresses and standards get superseded. Honda are just saying, as long as it's at least this good, then it's ok. You can use oil of a lower standard if you want, it's up to you. Really. But if you have an oil related problem, Honda won't help. Use the correct oil as specified and Honda's warranty holds. You could argue that when the bike's out of warranty it doesn't matter, but if Honda decided the bike needs that standard of oil to work as intended, that requirement will not diminish over time and let's be honest, Honda know quite a lot about engine design and lubrication requirements and ain't gonna recommend junk and they don't make oil (i.e. they're not an oil company and are not trying to coerce users into using their oil) so what they recommend is worth following.
Then there's the viscosity. In the Training Schools we had a hard time convincing dealers to not use and recommend 20W50 (a common oil available at the time). This thicker oil is NOT to be used at the average ambient temperature of the UK. In the Arizona desert, maybe. Cleethorpes - probably not. Testing (by Castrol I seem to remember) showed that at low temperatures, the thicker oil could take 30 seconds to reach the cam, after engine start. That's a long time for the cam to be running on just what's left from the last run. In the UK, 10W40 was the correct viscosity.
Nowadays, Honda have mostly lowered that to 10W30, probably sometimes with the same engine for which they previously recommended 10W40. The fact is now that oils have improved and even at high temperatures, a 10W30 oil can still maintain the required film over the important components and since it is actually thinner, there's less drag so more power and better MPG, both of which have become increasingly important.
So the use of 10W40 as opposed to 10W30 will not damage the engine. After all, at lower temperatures, they'll both have the same viscosity. It's just that when the engine's up to normal operating temperature the latter will end up a bit thinner. So back to the DN-01s HFT troubleshooting chart, there's no way those problems apparently caused by 'faulty oil' have anything to do with the use of 10W40 instead of 10W30.
So as long as an oil meets or exceeds the (API, JASO etc) standard specified by Honda and is of suitable viscosity, you're golden. Doesn't matter whose oil it is and in the case of the DN-01, car, motorcycle, synthetic or otherwise shouldn't affect it either. Would I use 10W40? Yes. I use that (Castrol motorcycle synthetic, I forget the name) in all my bikes and it is indeed the viscosity recommended by Honda for most of them. However I have no qualms using it in the more modern ones for which 10W30 is recommended. Once my DN-01 is complete it will also run on Castrol 10W40 synthetic and I am 100% confident it will NOT cause any problems.