Why is there oil on my floorboard?

I have been finding what looks like a watered down puddle of oil on my passenger side floor mat and can’t see where it is coming from. Looks to be from the underside of the heater box but I can’t make sense of that so I was wondering if the lines for the oil light run in this area and over to the indicator? Only happens after I drive it, don’t see anything while I’m driving, and usually only ends up being a few drips before it cools down and stops. Can anyone help me understand what the cause of this would be? It’s a 68 Dart with A/C, 225 Auto. Neither heat nor A/C in operation during this. Thanks as always!