B, I sure would not mess with any high octane fuel. That engine has a compression ratio of 6.2/1. Just barely enough to make 85 or 87 octane fire off.
Lead was useful to lubricate valve seats, not valve guides.
When it was rebuilt in the 80s, it probably got hardened seats.
I tore down my 8N and rebuilt it in the garage in the mid eighties. It has hardened seats. And it has run thousands of hours since pulling a plow, disc, corrugator, terracing blade, and fresno. It runs as well today, as it did with the fresh rebuild, all on 87 octane, no lead, 5% ethanol.
Have you taken the fuel cap off and seen the fuel boiling inside the tank from engine heat yet? That is when you know you have been working the old girl.
One cousin has the Furguson OHV twin to the 8N, a TO 20. Grandad had three 8Ns, an Uncle has three more, my Dad had one. They all get fed the same diet as mine. They all have spent 50 years working in the fields, and none have failed prematurely.
Don't sweat the fuel. She will run well on whatever you have available to feed her. And with your rate of usage, she will still be running when your grandkids die of old age.
