If your motorcycle manufacture recommends 60 weight oil, then 60 weight oil @ 302°F makes a film thickness to match your clearances inside your engine. These clearances are usually based upon mineral oil. Because synthetic has a stronger tinsel strength than mineral oil, a lower weight oil may be used, but only one weight down.
So for your particular case, if your manufacture recommends a 60 weight mineral oil, you may use a synthetic 50 weight oil or a 60 weight oil.There is nothing wrong with 40 weight oil, but it is too far out of specification for you to use safely.
To answer your question on engine life, most wear on engines happens at start-up. The first number on your grade will determine your flow when cold. A lower first number will flow better when cold than a high number. For example, a 0w60 grade oil will have a much higher flow rate when cold than a 20w60 grade oil (0w is actually 250 times thinner than 20w when cold, but has an actual viscosity of 40 and not zero).
The higher the flow at start-up will determine the wear amount. When your engine is at normal operating temperatures, there is absolutely no difference between a 0w60 grade and a 20w60 grade, they are both 60 weight oil.
Mobil1
15w50 is an excellent oil. I have read the MSDS specs on it and the viscosity indexes of Mobil1 are as good or better than even the next grade up.
(note: 2010 te450 specifies 10w-50 synthetic, therefor 15w-50 matches perfectly to the manufacture recommendation)
Hope this helps.