I apologize for the above, I chalk it up to the stress of the times, or whatever.
In any case, M1, 15W-50 is still quite a robust oil and it works perfectly in many racing applications. I'm a turbocharged engine guy and use it regularly. If you are worried about lower zinc and phosphorous (ZDDP) , don't be. The 15W-50 contains plenty to do the job, and a whole bunch more than more common automotive oils. Remember that 1000PPM is sufficient and any more than that is not "more helpful" . In this case, the job of ZDDP is to form a protective coating that exists only to reduce wear when metal to metal contact occurs. Viscosity of the oil must be sufficient to avoid that contact, lest an engine self destruct. Put another way, the additives are a part of the entire package of performance.
The M1 has a HTHS (cP viscosity at 150c or 302f) of 4.5. Quite simply, it retains excellent viscosity at very high temperatures. Furthermore, the oil's pour point is still quite low, at -39f.
That's enough information to tell you that the oil's base stock is a quality synthetic product that works under a very wide range of temperatures. If however, the engine you operate is air cooled, has very high oil temperatures, relatively poor cylinder dimensional control during operation and highly loaded components due to a series of mechanical compromises, you may in fact need a higher viscosity oil.
So if you are asking if 15W-50 is as capable a product under the above conditions, as M1, 20W-50 V-Twin, the answer is no. The motorcycle oil is even higher in viscosity, the characteristic that might be better for the above mentioned engine.
My Lycoming aircraft engine requires a 50 viscosity oil here in South Florida. Maybe even a 60 viscosity oil. Trying to run that engine on something like a 15W-50 oil would result in high oil consumption.