OZ,
"If we take Havoline Synthetic 5W40 which has 2100ppm of Zinc, does it mean I will have 2100ppm of some-type of Zinc (not original Zinc) after 3000miles/5000kms?
Let's say the virign oil has 2,100 ppm of Zinc Diaryldithiophosphate or Diakyldithiophosphate. I.E., ZDDP is an ester of zinc, sulfur, and phosphorous added to the oil.
Minus the evaporation of a small fraction of the ZDDP and plating, you might have say 1,895 ppm of zinc at 3k miles. Now if you extend the oil past 3k, the ZDDP starts breaking-up into its separate components so it's effectiveness is reduced, because it need the sulfur atoms to help the zinc bond to the metal. So you will have zinc in the oil, but the zinc cannot rebond to the metal as it did before. When you add oil, additive replenishment takes place and replaces
the bonded zinc, phosphorous, and sulfur atoms, so they can once again attach themselves to the metal.
In additive chemistry, we say that ZDDP is a polar ester, because polar generally means it has an ionic affinity to metal. Charges of different signs (+, -) attract each other.