i am willing to bet good money that the reason shell decided to invest in the JASO MA approval,was to get/maintain a larger market share.for many years,all over the internet,different people and sites have been recommending diesel rated oils for bikes.the reason this came about,is mostly associated with the extra cost of motorcycle specific oils and peoples desire and actually using PCMO.
in the dismissal of the belief that a MC specific oil is needed,diesel rated oils have been recommended because of their generally higher levels of additives,specifically,additives like phosphorous and zinc.this,however,is both true and false at the same time.i suggest to people to study some of the PCMO out there and their modern diesel oils too.i won't go into specifics,but there is more than meets the label and approvals here.
probably one of the most recommended PCMO for bikes,over and over again,has been mobil 1 15w-50 and with diesel oil,the rotella.the reasons for this is long to explain and both of these oils have a long history of filling many niches,in combination with,peoples desire not to get "fooled" into using and paying for a specific oil for their application.(there is some truth to this,especially cost)
the requirements needed for one of the JASO ratings,i'm sure would surprise people to learn that,it's actually not that difficult to get,especially if the oil was already formulated a certain way and already met other ratings.the JASO rating is not specifically about how much an oil will actually protect all the different parts of a bike.most of the requirements are actually similar to API,ILSAC,ACEA performance categories satisfied,but with two exceptions that will dictate were exactly an oil will fall in the JASO rating.
without getting into a long explanation,one of those is,dynamic,static and stop-time friction indexes(related to proper wet-clutch function).this is part of were an oil will fall in a JASO performance class category.they are MB,MA,MA1 and MA2.however,they can overlap with another requirement that is sulfated ash mass % 1.2 maximum and phosphorous max % that falls between .08 and .12(MA1 and MA2).
some of the traditionally thought of criterias for an oil that properly protects ALL parts of a MC to a high level,are suspiciously low in oils that are constantly being recommended for bikes,including those with a JASO ratings. a MC engines properly met needs,is not that difficult to achieve with a high quality PCMO,depending on what oil and type of application your talking about.the wet-clutch application,is a matter of specific friction index an oil has,...but the transmission and gears is were the proper and maximum protections needs,flys directly against the other two MC sections(wet-clutch,engine) requirements fall,..think about how manual transmission/gear oils are formulated.once the powers-that-be throw in emissions requirements,which they have,all bets are off.
MC gears don't have to out right fail for you to know there is damage,they can also chip away pieces through time.
it is all a big compromise and a huge,complicated mess.i for one,will always go for maximum protection while having no clutch slippage.i don't have cats and i can always take care of deposits.
now,i'm not saying that rotella is not a good oil for your MC AT ALL,..what i am saying is that,there's more here than meets the eye.i also suggest for you to study how many of the modern diesel oils are being formulated.there is also the issue of which specific MC in question and the riding style.i'm sure that many MC should be fine,others are probably being border-line protected.
inevitably,there will always be people who say,i have used X type of oil for X number of years,with no problems. that's all fine,i have no problems with that.i usually tend to believe them,but in the end,i will make the call on which oil to use on MY bike. in a way,the internet has been both a blessing and a curse with very good,not-so-good and false information,even on dedicated forums.i am not trying to condemn,upset or start an argument here.
....and then there's the question of moly and wet-clutch function which never seems to die,lol.i'll probably start a new thread for that.