Joe, Many of us have cussed n discusssed this very topic numerous times here or other places. For what its worth (it and 75 cents might get a coffeee if not at Starbucks) heres my thoughts n opinions, Im sure others will add theirs later.
1) For one thing, unless youre bored n stroked n domed pistons etc, those old tractor have such low compression they dont need any lead or lead additive to prevent pre ignition in the first place.
2) Some of those can accumulate and leave deposits n foul the plugs, especially as youre not working with any super hot spark energy in the first place.
3) On valve n seat wear and valve recession, unless youre using her for hot long hard continuous loads, its gonna take a long time to make much valve wear difference with or without any additive. The use of hardened valve seats and/or check the valve lash time to time could either suffice if you have any problem.
As an alternative and whats worked well for me for this and other potential problems is I add low ash 2 cycle oil to my gas for corrosion prevention PLUS its a good top end lube, then I use Marvel Magic oil in my crankcase (NEVER in my gas) as an addition top oil lube, plus I use Sta Bill, year round and check valve lash now n then.
Thats my story n Im stickin to it, I dont use it nor think its needed and it may foul the plugs. Hope others add their findings and opinions to this also then you can decide for your own self.
Good topic, good luck n God Bless ya
John T in Indiana