You can belend oils of different weights provided they are the same brand and style
The problem arises when mixing brands and types as some additive packages are not compatiable to othere
So you can mix any two ( or more ) Mobil 1 oils together without worry
But you might not be able to mix Mobil Super & Mobil 1
Now viscosities are climate specific
If you push snow then go for the 15W50 or even a 10W50
If you only mow then 20W50
If you mow in Texas or Arizonia then even a 20W60 would not be a problem.
Cub & JD both specify a wider viscosity range so they can use a single oil in everything from Alaska through Utah & right down to Florida
As hydros wear I have been known to use Harley 40W 70 oil in order to squueeze a few more seasons out of a mower that is not worth doing a $ 700 tranny repair .
Hydros are the one place where a synthetic oil is actually worth the extra money in a mower