Differential oil probably has a good dose of molybdenum disulfide powder in the mix.
This element is commanly placed into gear oils, chain lubes, and greases.
It can, and most likely will, make a clutch slip as it will tend to glaze the clutch plates.
However, the form of molybdenum (moly) we see in engine oils is different. It has been proven to not effect the action of a wet clutch to any great degree unless used in a very high concentration, and in conjunction with a very thin oil base.
When we see the API starburt on the back of an oil bottle that states "Energy Conserving" it only means one thing. The level of phosphorus has been reduced from previous formulations.
It also implies that the oil maker placed some sort of component in the oil that serves as an Anti-wear agent to make up for the loss of the phosphorus. (they may well not have added anything)
Btw..the EC rating and lowered phosphorus content is a bunch of hooey, if you ask me, as it is all because some studies showed that it could be detrimental to the catalytic converter of autos. However, unless your engine and cat are already clapped out for whatever reason, I haven't seen any evidence that there is much effect by raised levels of phosphorus. But I digress...
Some of the oil blenders were using moly as a replacement for the lowered amounts of phosphorus. The word "moly" for some reason became synonomous with the term "Friction Modifier". And it sems that anyone who reads the words "friction modifier" has it in their head that the oil has moly added to it and it will foul up the action of a wet clutch.
Not the case. Especially since it was not the form of moly (MoS2) that I mentioned above that is in gear oil and grease, but rather moly in the MoTCD form that is shown to be fine for wet clutch use.
And moly isn't the only additive that oil blenders use to make up for the lack of phosphorus. There are a multitude of additives that can provide proper friction modification and extreme pressure lubrication. Some of them are solids, and others are not. Sometimes an oil blender will simply raise the level of another already used component.
The dealerspeak BS and track myths that came about as a result of these oil formulation changes is amazing.
But the fact is that you can probably use just about any EC rated oil on the shelf in your dirt bike and not see one bit of a problem from a machine that is within specs.
Many see trouble simply because they have a machine with glazed plates, or sacked out springs, amoung other reasons, and when they try a different oil the machine may well act up...as the oil they chose may well have exaggerated a pre-existing problem. But, of course the problem is blamed on the oil used.
Oh sure, one oil may make shifting feel different than another, but still is not detrimental to the clutch action.
Friction modifiers are not something to be afraid of. Folks simply don't understand what the term really implies.
But some of the oil formulators who provide moto-specific oils...for often 5 times the price of their auto, or diesel oil on the shelf next to them, love the fact that the consuming public is ignorant when it comes to oil. The track myths and dealerspeak BS lends itself to them making higher profits from selling you the moto-specific oils.
Now, that is not to say that some moto-specific oils are not good oils. Not at all.
I happen to think that Mobil Racing 4T (formally Mx4T) is a fine oil and built as well as one can be, using top notch bases oils and good levels of much needed additives. BUT, there are oils on the shelf for much less money that have been proved to perform every bit as good as the Racing 4T, for FAR less money.
There are also MA rated moto-specific oils that are crap compared to others.
Jaso MA ratings are merely a guide for the consumer who knows absolutely nothing about engine oils. It allows you to pick an oil that has passed certain qualifications and tests that show it to be within a certain standard on static and dynamic clutch action.
But just because an oil does not have the MA rating, does not mean it won't also pass the same tests and qualifications.
Nor does an oil with an Energy Conserving starburst mean it will automatically make your clutch slip.
Btw..I don't think you will find that Honda says their MB rated oils can NOT be used with a wet clutch. They only advise against it. But I can tell you first hand that you wouldn't be able to tell the MA from the MB rated Honda oils in a wet clutch.