My guess is that the O.G.
Skyactiv engine aka the 2.0 came stock with the original part# PE01-18-110 spark plug....which is more of the Denso TT 4711 design BUT IS NOT the same part. Although they physically look the same the gap spec are different. The original part# PE01-18-110 had the smallest gaps (0.030—0.043 in) of ANY spark plugs for skyactiv engines.
....See I did not catch onto this until later on when people started pulling out their OEM plugs from their Skyactiv engines. Some had dual tips and some had one tip. Let me further elaborate. Okay Follow me on this.
Later the PE01-18-110 became obsolete (I say this because I can't find a dealership that sells it ) and were later replaced with other part numbers; the current part number being PE5R-18-110. Now the PE5R-18-110 if I'm correct looks like the NGK 94124. Both the current OEM part # PE5R-18-110 and the NGK 94124 have gaps at the wider end of the acceptable spec range (aprox. 0.044) for skyactiv engines. Thus are my opinions.
The PE01-18-110 should last longer maybe 80-100k miles. (Can't find these anywhere. Then again perhaps Mazda feels the later versions work better for skyactiv engines?)
PE5R-18-110, NGK 94124, Denso TT 4711's all have a shorter life span only due to their gaps being already wider when new compared to the original PE01-18-110.
Can't judge spark plugs simply by how clean or dirty it looks. Have to check if the gaps are out of spec too!
The NGK 94124's and the Bosch 9621's are looking appealing. They physically look like and have closer specs to the current PE5R-18-110. Not to mention cheaper.
Still debating if I should get the OEM PE5R-18-110. If so I'm getting it through Med Center Mazda.