'89 gmc jimmy with oil in floor

'89 gmc s15 jimmy w/4.3, 150,000 miles.
I noticed the other day that in the passenger side front floor there was a puddle of oil. The oil pressure is fine. The oil in the floor has just happened in the past 3 weeks or so. I know this cause with my side job I do once a month I have boxes that sit in the floor and they were fine last month, then this month when I took the boxes out one of them had oil on the bottom of it. A bad heater core would put coolant in the floor so it isn't that. Any ideas?

Hmm, really no oil anywhere near that area, boxes leaked?, it feels oily, which coolant does?

Hmm, really no oil anywhere near that area, boxes leaked?, it feels oily, which coolant does?

The boxes had paper in them and the bottom of the box was black when I picked it up out of the floor.

Pull the rug up and see what is under or in the carpet, so far sounds like guesswork.