I like how breaks or cracks cause the resistance to change, I've got an electrical trade and engineering diploma and a break in a lead = open circuit not a variable resistance. He sure must go through a lot of service parts over servicing? These engines have 2 spark plugs in series and one will fire even if the other plug and or lead is stuffed due to a capacitive effect so I doubt any slight increase in resistance within the service range is going to do anything to the spark or engine performance in real terms, maybe he's like a lot of others, who shove 98 octane in an engine not designed for it and think it makes tonnes more power.