What octane fuel for Briggs and Stratton Intek 24hp intek v-twin for Husqvarna Lawn tractor?

I have had a 21hp V-Twin B&S Intek since 2003.....the first thing I did on a recommendation of a small engine mechanic - was to get rid of the puny little fuel filter and replace it with a autoparts store big one - that has been on there since.....no problems ever - but was told it was a weak point. 87 - 91 octane has never seemed to make any difference....I use fuel stabilizer though....and need to check the valves soon....almost 400hours now.

I stock the Fram G1 fuel filter. 2 1/2 times the size of stock filter and cheaper too. That and a shut off valve went in on day one. JD 350 w/Kawasaki 18.5hp rated.
 
My tractor is surging at idle either because I used E-15 in it or because I used a bungee cord around the throttle lever to hold the side discharge open to mow tall wet grass in a hurry. The throttle cable is not making it to the snow icon anymore (makes it but won't stay there so I suspect it is that).

...but the experience scared me and I won't be doing either of these things any more....

So the manual says minimum of 89 octane. What would you do? I am not sure what my local gas station has...options are likely 87, 89, 91, and 93.

Any advice on octane?

Thanks.
Around here ethanol free is usually 90 or 91, so plenty high enough for the manual and you will get better fuel milage (GPH) as well.
 
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