I believe that most of us understood that leaded gasoline can lead to spark plug fouling.   It turns out even with the "Scavenger" compounds it apparently will still happened.  See article below:

 

I also read one report that address aircraft engine effect.  It basically said that if the CHT is not kept over 350F the scavenger effectiveness is minimal.  So this may be why aircraft sparkplugs do not seem to foul as often as the auto plugs (higher CHT).