Here are some of my opinions... since you asked.
I have decided to move away from synthetic in my '94 Harley Davidson (Evolution engine) for the following reasons:
1) The cost of a decent synthetic is getting too darned high; I had been using Mobil 1 "redcap" 15W50 but they've changed the formula and raised the price.
2) Dino oils are better than ever.
3) The UOA's at this forum, if read and understood collectively, show no advantage whatsoever
to running synthetic oils. In the cases where wear is not identical, it's actually common to see the dino beat the syn in that department--and low wear #'s are the "be all end all"--in the end.
4) In a motorcycle engine you're likely going to be dealing with either a sump common to the tranny and the engine, and/or an air cooled engine which likes to run on the hot side. And more fuel dilution issues on some models. And probably a higher tendency for the oil to collect "garbage" from the by-products of combustion. For this reason, quick drains are important. I have decided (remember that we're talking about my opinion here) that it's too risky to leave an oil in for more than 2500 miles on a road bike. I will probably change mine out at 2000 miles to be on the safe side. Again, issues of fuel dilution and other contaminant build up in the oil are too prevalent in these engines to go for a long drain interval--so why use synthetic?
4) For the issues mentioned in #3 above, it is not cost effective to run a synthetic oil--in order to justify the price, you've got to extend the drain. And if you extend the drain, you may be running a fouled oil; the only way to be sure is with a 20 dollar UOA. (Which defeats its own purpose).
5) The only real advantages to synthetic oil would then be its superior ability to stay in grade, and its ability to avoid baking into a paste if you over-heat your engine. (Notwithstanding the fact that if your engine ever gets that hot it's toast anyway!
) However--with quick OCI's using the newer dino oils, neither of these concerns are real enough to merit the extra expense of the syn.
But I know you asked about ash...
I think it's probably a non-issue. I'd be more concerned with TBN and viscosity retention than with the ash content. If ash was a "clear cut" concern, there wouldn't be so much conflicting information out there I wouldn't think.
If you can run a 15W40, I say go for the HDEO--it'll serve you well, I feel sure.
I wish Chevron made Delo in 20W50. They do make a straight 50 weight Delo which would probably be robust as all get out, but not so good in cold weather.
So we compromise. My HD calls for a 20W50, and I believe I've found the best 20W50 dino on the market in Havoline. It's still got the zinc/phos package pretty well intact, and the flash point is 492--better than some synthetics of that same weight class.
I do plan on a UOA of the Havoline this spring, after I've put a couple thousand miles on it.