got me looking at GTD's again http://www.ebay.co.uk/itm/1991-VW-VOLKSWAGEN-GOLF-MK2-GTD-TURBO-DIESEL-RED-/170690067886?pt=Automobiles_UK&hash=item27bdebbdae#ht_500wt_1156bit suspect about the 'oil cooler' leaking oil into the water probably a glown head gasket or crack between the oil and water ways.
Wouldn't worry about oil cooler just change it and do a coolant and oil flush and would be sweet..