Straight mineral oil had a lot of paraffin or wax in it and it did settle out over time. What you have found is very common. The black/silver is carbon, not metallic. This is also common and nothing to be alarmed by. I wouldn't even bother to remove the pan. The paraffin will become liquid when it is hot. Change the oil, run the engine hot and pull the plug while its hot and let it drip a while. You might want to then drive it a few miles and drain it again.