I always suggest that people do what the manufacturer recommends, unless there is some special circumstance. I don't see any special circumstance here with regard to the amount of oil in the fuel. (If you had a utility on permanent scrub-bashing duty on steep slopes with the governor removed and running flat as a biscuit, it might be slightly different, but only after you wore out a couple of engines and posted pictures of the worn out parts.)
So far as the fuel is concerned, I think there are advantages in 91 octane unleaded, with no ethanol. It is fairly friendly to your carburetor diaphragm and some other bits. If you can't get that any more, you have to decide between eating a few polymer components after a few years, compared with probably having more gum form in your carburetor than you are used to. Neither option is at all awful. Of the two, I think I'd rather have the gum than having to replace parts that will become increasingly scarce. Having said that, I should point out that because of the gum formation problem with all unleaded fuels, the major brands put an injector cleaning solvent into the fuel as standard equipment (they did that after some fairly heavy pressure from car manufacturers). The solvent is very chemically active: if you buy it as an "add this to your tank" substance, you have to take special precautions or it strips the tin plating off the inside of your fuel filler. The stuff is present in both 91 and 95 octane unleaded, but there is probably more of it in the 95, since there are more of the olefinic products of cracking, and they are the main culprits in gum formation. So, what effect does the injector cleaning additive have on fuel system polymers? I don't know, I was out of that part of the business before the results were in.
