Pre-85 W201's had a "live" oil pressure line to the gauge. In '85 it became electrical. I would pull the driver's side carpet and pad to get down to the floor pan and have a look at what's dripping/collecting. On the other side of the firewall is the master cylinder and brake booster. To the right of the gas pedal is the heater core in the center console, which might be leaking, but shouldn't be oily.

As seen in the picture, there is a hydraulic line that runs just behind the pedals.

Pedals