My '85 2.4D has black oil in the radiator but still runs ok. Is is possible for sump oil to get into the cooling system through the oil cooler or the welch plugs in that area. I suspect a cracked head but I could use some feedback before I remove the head unnecessarily.
Not sure about the diesel, but on the gas 2.4 (22R/RE) a worn out Timing Chain can wear a hole thru the TC cover allowing coolant and oil to mix. A blown Head Gasket will also do it.