I do a lot of suspension work. Most people don't think of suspension maintenance until the seals leak. All that black stuff is usually aluminum oxide, you know the active part on black sandpaper? Works the same. I've seen a lot of forks that have the slider worn out and no new bushings or seals will ever fix it. Buying suspension parts is very expensive. Seals, oil, not so much, but a slider can run into the hundreds. The old damper rod forks could drain out most of the oil. The cartridge forks are impossible to drain without taking them down. The 'friction modifier' added to limited slip rear ends is fish oil. The smell from gear oil is sulfur.