Having some 'time' on ageing 737-200s, it is well known for oil to seep out from the junctions on the engine cowling, especially towards the rear. It is NORMALLY just the normal 'gunk' that accumulates inside an engine cowling. The other thing that gets pax and other 'younger' observers excited is the (common) stream of 'smoke' from under the engine from the oil breather.
I will always remember (when I was a keen young thing) popping up to the flight deck after landing to tell the elderly captain that there was oil seeping out of the rivets on the No2 engine on his DC3, to be met by the gruff response " at least that means there is ***** oil in the ****** thing"