I think the deposits left on the "flat surface" at the end of the threads is oil and fuel additive related.
Using the same jetting combo, if I ran Citgo sea and snow or Repsol synth2t, the "flat surface" would be brown. With Yamalube 2R, same jetting and mix ratio, the flat surface is totally clean. The piston top is also totally clean, with only a small heart shaped carbon deposit.
The center of the electrode is dark brown. Bike runs perfect.
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'10 EC300 -- very happy 
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