Typical problem with these cars. Usually only a head gasket unless it has been overheated. Mine is using more coolant than I would like, and I have oil in the coolant tank, too. No trouble, even on my trip to Florida, but I still believe I have a head gasket in my near future!
The original head was trouble. The re-design replacement is much better, but not cheap.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!