BMW put a nice little chart in the owners manual.
I follow it.
Its all about the year and the bearing clearances, depending on how the rod and main bearing clearances were set and the amount of oil pressure and I suppose the phase of the moon, the viscosity affect the actions of some of the internal parts like tensioners etc.
I was told once that the first number on oil is the equivalent viscosity at 0C and the second is at 100C.
So 10W40 is the same viscosity as 10 weight oil at 0 and the same viscosity as 40 weight at 100C so I was told... Guy was a Chevron engineer...
Dave