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Per Caterpillar, high levels of copper found in oil analysis are a result of engine cooler core “leaching”. The zinc additive in diesel motor oils reacts with the copper cooler core tubes at high oil temperatures. This chemical reaction results in copper oxidation products forming on the surface of the tubes and then coming off into the oil as it washes over and around the tubes. Heat is a catalyst to this chemical reaction- the higher the oil temperature the greater the rate of oxidation and the longer the reaction process is sustained. This does not cause any damage to the cooler core, or to the engine. However, you will see the copper reading elevate to over 100 PPM. Copper is not a particulate in this case but is in solution in the oil. The PPM will start relatively low and increase over 100 PPM and stay at high levels for several intervals, and then return to normal. During the oxidation process, a hard film forms on the tube surfaces exhibiting the appearance of clear varnish. Changes in operating temperatures, and /or changing oil brands can chemically disrupt this film and begin the oxidative leaching processes again. This can occur in any system with a cooler core: Diesel or gasoline engines, Transmissions or Hydraulic systems.