Here's a link to the SAE paper that lead to the JASO MA spec for motorcycle oils:
What's most interesting in this paper, to me, is not that moly can cause problems for starters and clutches. Instead, I'm thrilled to actually learn what viscosity oil the Japanese manufacturers use when designing and testing their motors!
I've been wondering for quite a while whether the manufacturers are aware of the rapid viscosity loss that almost all oils undergo in a shared sump environment, and whether they planned for that when recommending an oil viscosity and change interval. I mean, several manufacturers are claiming you can go 6,000 miles between oil changes, so surely they'd design their bikes to run just fine on oil that has sheared a grade or two, right?
Well, not really, according to this paper. It's true that most manufacturers use a 10W-30 when designing and testing their engines, and I infer that their motors would run just fine on 30-weight oils. But, they've also seen significant pitting of the transmission gears when running 30-weight oils that are on the thinner side of the grade.
This is most likely why the manufacturers recommend 40-weight and 50-weight oils--not because of the engine, but because of the transmission. This makes sense if you look at the comparative viscosities between engine oils and gear oils. The most common gear oil weight is a 90-weight, which runs between about 13 cSt and 24 cSt at 100 C, while a 40-weight motor oil can be as thin as about 13 cSt and a 50-weight motor oil can be as thick as 22 cSt.
So, the most optimal viscosity oil for a motorcycle with a shared lubrication system is likely to be the one that is as close to a 30-weight as possible while still protecting the transmission gears from pitting. This will vary from bike to bike, but you most likely do not want to let your oil's viscosity drop down into the 30-weight range, otherwise you risk damaging your transmission gears.
My take-away from all this is that I'm on the hunt for the most shear-stable 40-weight oil. I don't want my oil to drop down into the 30-weight range, and a 50-weight oil causes a lot of parasitic power loss, so a 40-weight that can stay a 40-weight over the entire oil change interval would be ideal.
Of course, some bikes require a 50-weight oil because of temperature concerns, so don't forget to take your expected ambient temperatures into account. Similarly, if you live somewhere near the equator you could just run straight 40-weight and not worry about viscosity loss at all. But, for me and my climate, I need a multi-weight oil.
I'll be trying Redline 10W-40 in my bike next.