Hi PlainSmart, Buzz here. Well, the fine wire plugs tend to foul less: i'm thinking of 65-100 hp engines on an Aeronca Champ that were made to run on 80 octane avgas... 100LL has 4 times more TEL than what is required.
I didn't see much of a difference on that airplane, we had some fine wire plugs and decided to try 'em.
Normally we see a little bit of "junk" (combustion deposits) deep down in the sparkplugs. But it doesn't cause enough fouling to make a bad mag drop during run -up.
We use massive electrode plugs on the DC-3 i play with, they're about 1/3rd the price of fine wire plugs. It makes a difference when you need 28 sparkplugs per engine.
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by choice, taildragger pilot for fun