Here is my reasons.

First, who is JASO? or what are they?

American Petroleum Institute

Both companies grade and certify motor oils making sure that oil meets certain specifications or requirements.

My take on these two different organizations.

1st the API. It's a US based company. It's governed by US mandates (laws) including the EPA and US industries'.

2nd. JASO. This company is Japan based, and I'm assuming they have similar mandates and laws and so on.

Now moving on to motor oil.

I said I don't like to mention sulfated ash either, why? because it only adds confusion to an already confusion issue. A topic will be going along pretty good and then someone says, yell, but that or this oil has Sulfated ash in it of whatever percentage. When that happens doubt feels the air and if someone tries out this sulfated ash oil, that doubt can? play a big part in their belief in the oil. Doubt can??? make us hyper-alert.

What is sulfated ash?

if you take an engine oil and a certain amount of it and set in on fire and burn everything that you can get to burn and keep doing this until you have nothing left but dry burnt crusties, then you will be looking at Sulfated Ash.

Guess what, you will find it in every motor oil you burn off.! That don't make it a bad oil, it's remains are not healthy no! but it's the additives that don't burn. It's these additives that make oil even work at all for the purpose we use it.

Here is an example,

Mobile 1 15W-50 Gold cap is a premium oil, it's a good oil, but what's it's sulfated ash rating (content)?

I took the above screenshot off Mobil's web site. It looks to be 1.42%???

Wow! now I doubt that this oil is any good! if I use it my valves will get deposits on them and fall out on the ground.

The Shell Rotella-T 5W-40 oil is getting a lot of attention but, it has a Sulfated Ash rating of 1.47%. Is it better or worse than the above Mobil 1 oil based on the ash levels? I don't see a difference between them. I see one oil as one that is made to work harder at cleaning, anti-foaming, and with more ZDDP than the other, but which is better?.

I know of some people that buy oil based on a low sulfated ash rating. Yes you can do that, but it will be a trade off for something good about the oil.

I personally know of a person that changed from a Low Sulfated ash dino oil to a full synthetic higher sulfated ash rated oil, what do you think came out of his engine a few thousand miles later when the synthetic oil was drained out? my take was it came out nasty. That's a sign that the other oil may not have being doing a great job of cleaning.

Another person I know was looking for an oil for his bike, he found the sulfated ash ratings and didn't like the sound of the valve deposits that It Could Lead To so the oil he ended up with was the same oil that the other person switched from. I'm still not sure about that but he is very happy with his choice of oils and it was a good oil. (or is a good oil)

Stick with me, I'm trying to shorten this really.

Here is what I'm getting at. When JASO pops up in a discussion so does the Sulfated Ash, it is a hand in hand bag of bee's. But if you take JASO out of the picture, then sulfated ash doesn't make much noise.

Here is my thought about JASO for now and it's not nice, 

but they can take a "Flying Leap At A Rolling Donut" I really don't give a rat's a*& about their ratings. 

It's a Japanese based company looking after Japan! There isn't one thing that is in oil that the API don't have at lest as good of understanding of as JASO does and that's including sulfated ash, glad I got that out. Now I would change my mine if Honda said something like this,

Your motorcycle is built in a way that it REQUIRES motor oil with A JASO MA rating! oil's with other ratings may be used but only if that oil also carries the JASO MA rating. 

I doubt any of us see this anywhere in the near future.

JASO also is fairly new at what they do as compared to the API. It's my understanding that when JASO became established that they just started with where the API was at that time.

If I remember correctly, give me some room I am human, but in 1994 JASO started a study or search for a way to reduce the ash levels in two cycle oils. That is oil that is made to burn. Then I believe in 1997 JASO set a standard for 4 cycle motorcycle oils, in that is where you'll find the "No More Than" 1.2% sulfated ash! but like I said that is in Japan, I live in the US. I wonder how many more motorcycles they have in Japan as compared to the US? any body got any idea? How many of those little 4 stroke 100 mpg two-wheel critters are running around over there to cause JASO to set separate standards for both Two-cycle and four cycle motorcycle oils? When I think along these lines I understand why there is some differences in the ratings for oils between the API and JASO.

So how about this question I get,

But MarkC I have a Japanese motorcycle! LOL My reply would be,

I doubt it, if it's a later model of Shadow then it's built in the USA! yep, it's owned by a Japanese company. These bikes are made in the USA! made to meet USA standards, emission's, sound DB ratings and whatever else. They're built to use US fuels and oils. Okay it might be great if the oil you and I buy would meet both API and JASO requirements, but either classification could mean that both are still premium oils. An oil carrying both classifications don't mean its a better oil. To me it's neither good or bad, I'm sure that either standard could be met with cheap oils. The one thing I see is that if an oil meets both then that oil could be sold in at lest two different countries from the same batch of oils. That's probably great for the oil companies

MarkC.