2000 Audi A6 Quattro Base 8 Cyl 4.20L
slow loss of coolant no overheating topped up with water
on last occassion stopped to top up expansion tank and noticed thick sludgy oil.
Jay Giron 11/28/2011
Usually a blown head gasket but if you have an automatic transmission and the tranny cooler is built into the radiator, it can also be a bad radiator that is mixing transmission oil with the coolant.