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Mineral oils may react with the copper conductors in oil impregnated transformers, leading both to the formation of dissolved and suspended copper in the oil and to the deposition of copper onto the paper surface of the solid insulation. Available standard methods for copper detection in mineral oil may not be fully effective in assessing the total copper content. Dissolved and suspended copper species in the oil are shown to affect oil's losses to a different extent. Unused oils are investigated to elucidate their tendency to dissolve copper in the oil and to deposit it onto the paper surface. Oil's inherent properties, the presence of additives and the reaction conditions may significantly vary the magnitude of the copper dissolution, as well as the transfer of copper to the paper tapes. A tentative relationship between the deposited copper and the paper's dielectric properties is described.