The oil cooler is positioned on the DS right under the DS exhaust manifold, a very hot location for a "cooler". The O-rings get cooked, turn brittle and leak. When I changed mine the o-rings fell apart when I pulled them out.
Obviously someone that towed often, or had a heavy right foot habit, would generate more exhaust temp more often.
I don't so my first change was at over 200K. Likely if there were a heat shield between exhaust manifold and cooler they would last an average users unit lifetime.
The coolant system cleanup after the repair took longer that the physical cooler o-ring replacement.
Good practice to change oil after then run it 500 miles, get an oil sample to confirm no water in oil.