PB,
the fact you use 98 in your lawn mower is hilarious (i too am guilty!)
The fact of the matter is, on crappy, small engines such as lawn mowers and the like, the ignition timing is fixed, does not even advance or retard and is factory set for rubbishy regular unleaded. So in actual fact, absolutly no benifit is gained from using ultimate in such engines, other than perhaps it running cooler.
Does that stop me ? Hell no.
I used to run my old lawn mower off a BBQ cylinder (lpg) a 9kg cylinder would keep the lawns mowed for about a year.