When I use to deal with small engines, I mean small like on Argos and smaller bombardier vehicles the biggest killer to an engine was "regular" gas. Maybe it has to do with tiny welsh plugs on the carbs and tiny jets but you could always tell the difference between two identical carbs run on regular vs premium. Reg carb would be varnished and fuel tanks full of sediment. Prem carb would be much cleaner and significantly less varnish on welsh plugs and jets and floats and the fuel tanks had less sediment. Oh, yeah if they are the same why do they taste different, additives? nah
The mid grade makes sense, again, higher octane rating, less detonation. Could very well be a sweet spot or "room for error" thing with varying quality of fuels between countries. Again, when detonation occurs and timing needs to be retarded fuel no longer burns properly due to conditions in the chambers and you begin to get more carbon build up due to lack of heat and lower power as a result. And less scavenging of the exhaust which means moist cool exhaust sitting in your exhaust system longer leaving even more deposits.
As I pointed out earlier I was over the top in my post(except on the fuel cost over time, that was fact, oh and engine replacement costs less labour and incidentals, you got the point though) for the simple fact to prove a point on the effects of risk versus gain. So the guy saves $520 a year going regular, wow, clap, clap, clap...guarantee him he will never damage his engine running regular it's entire life, however long, or short, that may be...the owner manual says premium.
My wife's Alero says regular. 200cc's larger than my engine and 50hp less, hmm.