I am new to PCP airguns and cleaned the barrel in my Diana Skyhawk today after putting approximately 300 pellets through it. I was amazed at how BLACK the cleaning patches were. I would expect that kind of filth in a powder burner, but not a PCP rifle. Is this normal? Is it just lead residue? Is there something I should be concerned about? Is it possible that dirty oil from my Yong Heng compressor works its way into the reservoir and then into the barrel?