I have black oily sludge floating in my coolant liquid resevoir.The vehicle is a 1986 300TD diesel station wagon.

Thank you Derick

Odds are.......head gasket.

Yup,
Sounds like your head gasket. And if you have oil in your coolant, then you have coolant in your oil. Do a search on the subject, you find quite a bit of info on the matter.

Change the oil, while it's draining, see what comes out. Usually as a rule the oiling system runs higher mean pressures than the cooling system. So, oil in the coolant is more frequently seen. Drain and flush the cooling system, use a mild detergent. Benzmac suggested "Shout" to me a while back. Have your compression checked. See if it recurs. Also, if diagnosis remains obscure, have the trans cooler tank in the radiator pressure tested. They can leak.