Had the same "leak" on the 4.5 a while back. One evening I was perusing the parts inventory on PartsShop (OK, I was bored) when I found replacement oil cap seals. Cost a whopping .30 cents, changed the seal and the seeping leak on top of the valve cover was gone.
You might be able to correct this on yours for even less than was mentioned, if the seal is available for your cap.
'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.