NO

add a little stable

No and No. Keep it in good tune, fresh oil, fresh gas and spark plug and it should last 30 years. NO FIX IN A CAN

What if carbon build-up in the combustion chamber raised the compression ratio? Would this not technically raise the octane requirement?

I was thinking the same thing.

Or what if normal wear lowers it? Run it on the lowest grade possible and add Stabil and you will be fine.

The argument is academic because you probably would not notice any symptoms in a mower engine from using gas with an octane rating lower than the engine should require. The other thing is, what is the actual octane requirement? I would assume it is lower than regular gas, so 87 octane might already be more than is needed. Use of fuel with a higher octane rating than called-for in the past was said to cause carbon deposits because it actually burns cooler. In today's lean-burn engines I'd say going up to mid-grade would probably be a good idea.

"Some cars may need higher-octane gas as the years go by... what about lawnmowers?
Has anyone experienced an issue with lower octane in their older equipment?"

No to both.

Using lower octane will cause knocking just like in cars. Using higher octane does nothing. (Unless you go to extremes and use aviation fuel. In that case it can cause overheating.)

I guess I have just been lucky to never have owned a mower engine that had preignition (knocking) issues with lower grade fuel, maybe the Stabil is helping in some way as I use it religiously? I would agree though, if you have preignition problems then a higher octane rating would be necessary.

Mowers are so noisy, how would you even know it had pre-ignition? That's what I meant by 'would not notice' above. Sure, it could happen, but would you know it? Has anybody actually identified 'pinging' in a mower engine? I doubt it, bacause the actual octane requirement is most likely lower than today's 'regular' gas.

Although octane requirements will increase with age, partly due to carbon build up increasing compression, and party due to sharp edges of carbon glowing incandescent when heated, you'll still be fine with regular grade pump gas.

Most flat head (L-head) engines are built with 6:1 or so compression and designed for an RON of 75 to 80. This includes Briggs Classics and Quantums, Tecumsehs, and older Honda, Kamasaki, and Robin flat heads.

More modern OHV engines are general designed to 7:1 or 7.5:1 compression, which helps make them more efficient. The OHV design also has some quench which reduces preignition and is less susceptible to carbon build up. Usually they require RON of 80 to 85.

Unleaded regular is RON 91 or (R+M)/2 87.

Right. So unless you are using ten year-old gas, you cannot buy an octane low enough to induce pinging, and regular is already higher octane than it really needs.

'Using higher octane does nothing'

Well, it does what its supposed to do, which is burn cooler and prevent detonation, thereby releasing more power than a fuel that allows detonation. Its the cooler burn which may cause carbon formation, but again, in today's lean-running engines, you might get slightly cooler temps without extra carbon. I started using mid-grade for my OPE the last time I got gas, and I've been watching the muffler outlets for increased soot, which I have not seen. I can't honestly say that I can point to any real reason for using higher than Regular in OPE, but sometimes I think the stuff runs better :)

Higher octane gasoline doesn't burn cooler. It burns slightly slower preventing detonation. In extreme cases like using super high octane avgas I have seen engine damage from superheating in a Briggs and Stratton.

I have had a lot less starting,and storage troubles since I began using a capful of MMO every fillup. The engines seem to run better too. MMO is a top cylinder lube and will gradually clean out carbon. I s'pose it slows down the burn because it is less volatile too.