2005 Polo SDI Oil cooler replacement.

[HD54]

Hi there, my 2005 polo SDI has what seems to be a failed oil cooler. Oil leaking from and filling up the expansion bottle. It runs perfectly, no overheating or strange noises. I was just wondering how the oil cooler came off. There is a large hexagonal bit underneath it, is it removed with that or are there other bolts etc to remove? Also, with the oil and water mixing, how best do I clean the coolant system and the engine as I presume there must be coolant in the engine now too. What oil is the best kind to use? What flushing products or additives should ii use to clean it out? I have been only able to get 10/40 oil but i was going to put an additive in to thicken as nowhere does any thicker oil. Any advice/help would be greatly appreciated.

Thanks in Advance.