Tournament Striper fisherman...... lots and lots of cold water (< 50F) trolling.
My 1997 200 HP Ocean Pro (new power head in 2010) trolls 8-10 hours a day, all day, any day and has never fouled a plug on me.
I run nothing E10 and semi-synthetic oils. Pennzoil Premium Plus, Evinrude XD50 or...