Only Champion or NGK for me. As a seasoned professional lawnmower trash / curbside picker and guy to go to when people are too cheap to pay a real lawnmower mechanic. I would never use E3. The core seems very cheap and I feel they run hotter than spec based on teardowns. I have seen about half a dozen Briggs OHV engines with blown head gaskets. In all cases, there was an E3 plug in the head. I had one with a burned out magneto and it had an E3. The fact E3's are made in China is a big turn off for me too. IMO E3's are overpriced garbage. Like ARCOgraphite says. A spark plug should last the lifetime of the mower/engine. My personal OPE only gets a plug change when they start showing symptoms. My JD130 garden tractor was bought new 20 years ago and has over 600 hours on it with the original plug. Spark plugs are the most over changed and unnecessary part people change on their small engines. When a small engine acts up, the first thing people do is change the spark plug, when 90% of the time its a fuel issue.