I had the exact same thing. Fortunately, the problem was the fitting on the oil cooler itself had a hair line crack in the female part where the ball of the line seats. Diagnosis involves removing that end of the line and taking a really close look. You may have to remove the cooler to get a good enough view.

You have 2 choices if that is the issue.

1) source another cooler either new or used and install or

2) find a replacement fitting. There is info on line re the number for the repair part. You will have to cut off the old fitting then drill and tap the hole. The drill and tap are relatively expensive for a 1x use so consider that in your calculations.

Others have said that the required fitting is the same that's used on the oil filter housing and a yard may be a source. Of course, you need to go to the yard, find and remove the fitting which may be more of a pita than it is worth.

Edit: I just re-read your post. If the line itself is leaking, replace it. I remember reading that the top line has to come out to remove the bottom. But that may be on a 126 only.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.