My employer creates and manufactures the additives that are blended with base oils to create the lubricants that we find in various retail venues and as factory fill lubricants in millions of cars and trucks all over the world. The additives for automobile and diesel engine oils require testing required to meet API standards, and that testing is both complex and expensive. The standards for motorcycle engine lubricants are different, and for good reason.
I spent 18 years formulating axle and transmission lubricants, which have different needs and chemistry, but only a floor away from the engine oil developers.
As a few on the thread have commented, the "winter" designation of 5 or 10 is of value only in cold climates. The main viscosity grade designation of 40 is the more important number.
The prohibition of "molybdenum-based" additives in the BMW standard is wise. There are a number of very good moly-containing lubricant components, sold by reputable manufacturers, and created by chemists to do wonderful things in very particular situations. But, if BMW says "no" then it is important to follow that guidance. Chemistry is mysterious. Certain compounds are very attracted to metal surfaces, and may out-compete other compounds to the detriment of the hardware. Formulations that bring in components, structures, or elements that upset the balance of extreme pressure protection, wear protection, friction control, friction durability, oxidation resistance, etc. will do more harm than good. Many of us have seen the "snake oil" testing favored by the infomercials or youtube generation, and rightly have skepticism. One such substance you all may remember is when teflon was thought to be good for lubricants. It is slippery, don't you know, so it must be perfect as a lubricant component. Just pour some in. More is better. Not always.
My approach, as an experienced formulator, is to trust the OEM products. What formulators realize after a few years of trying is that you don't know what you don't know. An engine oil or axle lubricant might meet the general industry standards. It might even go beyond those standards. But, most often the formulators have limited access to the really important tests performed by the OEM on their vehicles. For instance, there is a desert durability test in NW Arizona, right near where the aliens landed and a small museum exists today. (No, I am not kidding. I have been there and there is a museum.) All the axle lubricant tests known to the industry can be passed, and yet the desert durability test failed. That test is secret to the OEM, and requires certain chemistry in the lubricant. The standard industry tests do not require that chemistry. Of course that OEM-approved chemistry passes the standard tests with ease. A desert durability test failure is catastrophic -- metal parts break. So, absent the knowledge that a lubricant has actually been tested by the OEM, I choose to trust the OEM-branded engine oil for my K1600. Doubtless other products can and do work, and many on our forum trust those products. Being risk averse, I try to avoid the potential failures of formulations that were not officially evaluated by the OEM.
What some OEMs do, in the interest of the end user, is to ensure that oils meeting a certain specification will work. Car engine oils, truck engine oils, axle lubes, all have the potential of meeting OEM tests, and most often the OEM designs the hardware to work with the oils passing the standard tests. Here BMW does give that API SL and JASO MA2 guidance, which is sufficient to define the oil performance.
Again, there are many engine oils that are not OEM-sanctioned that do perfectly fine. The formulations may even do amazing things, like reduce the clunkiness of the transmission shifting, or have lower wear metals as measured by the test labs to whom we can send our samples. I am NOT criticizing any other products, or suggesting that anyone who uses them is making a mistake! I'm just explaining how I approach lubricant choices, based on personal professional experience.
