Dark black fluid coming from under the dash, driver's side. I'd bet money the fluid is real slippery too. What it sounds like is either the oil pressure gauge, or the oil line fitting is leaking. Could also be the oil line has been rubbing on something under the dash and worn a small hole in the line, but more than likely the fitting is leaking.
To check and tighten you'll have to pull the instrument cluster to gain access. This is a W123, correct? I believe the cluster is only a pressure fit, pull outward gently. The oil pressure line fitting should be a 10mm, be sure to use two wrenches, one on the fitting and one on the gauge.
73 280 SEL 4.5