I've got a 1.9tdi 2006 a4. Loosing some coolant, and have got oil floating on top of the water in the header tank. Changed the oil cooler but the prob still persists. Oil is fine on the dip stick but the oil cap seems sludgy underneath as of there is water present. Car runs perfect, no over heating or smoke. Still very responsive. No idea what's going on! Any ideas guys?