Zero piston slap noise. less blowby, less oil via pcv valve. Less to zero "tick tick" noise when driven hard. Better oil pressure when hot. i can go on. Hybrid starting kocking noise went away on my ct200h when i did use m1 0w-40. And you are not forced to use a 0w-20 in a Toyota in my country even within warranty period. And Toyota owner manual says to use higher viscosity if you drive hard or do much highway drives.
The part when i said it runs better i cant explain it on English so... you have to figure it out.
Funny. I have none
of those issues running the spec'd 0w20.