What was interesting about that link is near the end, where the OP describes his initial assessment, the leaking rectangular O ring for the housing that the oil cooler lines attach. Probably the only exterior O ring that makes sense for an oil leak in that area.

I am surprised that O ring is included in the oil pan kit, as it is external and can be replaced with the left front wheel taken off. A jack stand under the sub frame of course.

I think in your car, 2001+, the oil cooler is not part of the radiator system but a separate entity.

Synthetic oil will slow down the progression of sludge buildup, but it is really driving style. Many trips are made to the closest grocery store, never giving the engine oil time to fully warm up. When my daughter commuted to highschool, I encouraged her to drive in 2nd gear for the 2 mile trip.
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My back feels better when I sit in a Volvo seat