This may not be related to MB cars at all, but it made for a weird repair. I once repaired a woman's Geo that was leaking transaxle fluid/gear oil out of the speedometer cable. A small carbon seal had ruptured in the transaxle allowing the cable attachment to be filled with oil. The spiral spring cable acted like an axial flow pump and pumped it up to the back of the cluster. It dripped out all over her legs and the drivers side. What a mess that made. The dealership wouldn't touch it.
If it is black oil more than likely it's the pressure gauge. If it's red, it may be another freak cable leak out of the trans. Belive it or Not!