[Walt Fricke]

Tightening the large nut is sure worth a try. The usual issue with these fittings on the thermostat is that the nut is frozen to the aluminum threads. If it is a bit loose, and leaking, perhaps the threads will be lubed too. Obvious place to start. Can't say I have seen or heard of this nut backing off, but why not try a wrench on it.

That nice drop of oil under the nut sure points to that location. And ultimately, cleaning everything and then seeing what happens is the normal way to chase leaks.

What to make of the brass colored ring where the line meets the nut? The line has a ball end, which fits a concave socket on the thermostat. The brass showing is not a consistent ring in the photo, but more of a semi-circle. Indication the nut is loose somehow? Could there be a crack in the line at the joint (never heard of that here or elsewhere).

The most common leak in the cooler line is from where the rubber hose part from the oil pump connects to the metal line. I assume that the grime doesn't extend rearward all the way to that point.