If the black oil from my engines had any metal in it, my engines would all weigh about .5 grams by now. It's just burnt oil, it is very common with piped engines and I get it with UltraThrust muffler equiped engines as well. I freaked out when I first saw black oil down the side of my planes, but it's nothing to get into a lather over. And FWIW, I ran true ABC Rossis for years with black oil spewing out of the pipe, and all of those engines are just fine.

If it's just the color of the oil, I wouldn't give it a second thought. If you hold the oil up to the sun and see sparkly bits, then you worry.