The big problem with small engines, used on lawnmowers, weedeaters, and snow blowers, is that they are used for a few hours, and then they sit, sometimes for months, before they are used again. Eventually, varnish and crud, impurities in the gasoline, begin to accumulate in the carburetor, and the engine won't start, or it might start, but it won't run well. One way to reduce this problem is to use the best gasoline you can find, & many people are using high octane gas in their small engines. It does seem to help some, and no one, to my knowledge, has ever damaged an engine with high octane fuel. It also helps to use a fuel preservative in the gas tank, but most effective way to keep the carb clean is to drain it dry at the end of the season.

It is a waste of money to use high octane gas in a low compression lawn mower motor. Just follow the recommendations in the owners manual.

you wont damage the engine in any way but you might cause a small drop in power with 91 compared to 87 due to the 91 octane burning slower and in a low compression engine like mowers have it is not needed almost every mower manufacturer i know of reccomends 87 . it also will not keep your carb any cleaner than regular 87 will because i believe it is federal law that oil companies put the same additives in every grade where it used to be that 87 and 89 had fewer additives so they could market the higher octane gas . for example shell used to only have their V power advertised in the 91/92 range now they advertise it in all grades so its the same gas just different octane ratings. 87 burns faster and hotter 89 is in between then 91 92 93 and 94 burn slower still . as octane number goes up the slower the fuel burns

I use 87 and a cap full of MMO with every fillup in all my OPE. Way less carb troubles. Mebbe the MMO raises the octane a bit.

I use 87 in all Small Engine applications , add Lucas Top End Lube and Cleaner on the 4-Cycle Applications and Regular Fuel Cleaner Treatment on 2-Cycle Applications to help in both Valve and Ring Deposit Reductions .