Oil cooler
When doing a job like i am doing right now (overhall cooling system & injection system), it's a must to change the oil cooler O-Ring wile the coolant and the water pump are out. So wile you have the coolant out (difficult to remove when attach to the 2 small oil coolant hoses) change the O-ring. I heard horror story about this one failing on the road.Note: the original O-ring was a “recall” by VW, it as a tendency to crack or fail at cold temperature.
Location of the Oil cooler

To remove the cooler you will have to drain the oil and remove the oil filter. Take a 27mm socket to remove the bolt (located between the oil filter & coller) holding the cooler (this is messy job, oil and coolant will be spill for an hour!!!). Clean everything and check for dent or corrosion. If you are equip to do a compression test, do one now. If you’re not equip, check this picture, it’s an easy way to do a test. (Rear heater section for more info)

The oil cooler with the O-ring, view from the engine side Note: the new updated O-ring as a silver / grey color. (this one is black, old & original one)

View from the oil filter side Note: clean both inlet & outlet (corrosion)

VW recommend the use of a sealant on the O-Ring side???, not sure about that. What I did is apply a small coat of Red 518 Loctite . Toque back to 18 feet / pounds.