Thanks for not "cheaping out" on your car. We read WAY too many posts asking if it's OK to use regular in "premium" cars.
The higher octane will not do any harm, but it sure is a waste of money using 91 octane gas in a lawn mower. Please note, the higher octane also will not do the lawnmower any good. It won't run better on 91.
I suggest the inconvenience of a separate transaction might be worth it. Why waste money on a lawnmower?