I agree that either the feed line for the mechanical pressure gauge, or the gauge itself has failed causing the leak. It is possible it could simply be the fitting needs to be tightened as well.
If you remove the cluster to find the source of the leak, be careful not to pull out too far before disconnecting the oil line. Also, when loosening or tightening the fitting, be sure to use two 10mm wrenches, one to loosen the fitting and the other as a counter on the gauge body. If you don't do that, and there was nothing wrong with the gauge, there will be afterwards.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.