While backing my truck I noticed I was leaving a trail of oil all over the ground. I shut it off as soon as I saw the leak and sprayed the street, sidewalk and driveway with oil. Some quick investigation found an (original...from 1978
) oil pressure switch that was leaking oil past the the two terminals on the switch. I unscrewed the switch, cleaned it off a little, and blew air (with my mouth) through engine side of the switch, and I could definitely feel that the switch was not holding pressure. That part of my Straight 6 has been oily for years, but i always assumed it was from the valve cover.
I caught it before I destroyed my little 6-banger, thank goodness.
The moral of the story....check your switch!