Sorry - I am confused - that part number posted is for the gasket for the oil cooler - from what Justin has said you will only replace that if the oil cooler housing had issues - repair/replace if it is broke and is not part of the regular service regime.

Or are we actually talking about the big seal around the oil filter housing that screws into the oil cooler - you know the one you get with the oil filter when you buy it - it is a few inches in diameter, looks like a big O ring but is square in cross section and when in place the corners of the square cross section rather than the flats stick out to seal the outer oil filter housing. Was it this seal that was left off?

Garry