C15 oil in coolant

2004 KW T600 with a C15 Acert-
There is a truck at the shop with an oil in coolant issue. Another tech just did the oil cooler, and in the first drain during flushing the emulsified oil/water mix came out as expected, topped by about a cup of nice fresh engine oil.
Now, with a new oil cooler, where does this leave as an oil intrusion point? Is there an o-ring in the head or something?

 

I'll have to explore that potential, although the engine was three gallons low on oil when it came in. I will pull the trans check plug tomorrow and see what's in there.

 

Local haul fleet trucks, lack of maintenance is to be expected.

 

I'm not sure about leftovers, this truck came in with a complaint of the heater not working. The heater didn't work because the cooling system was full of the nastiest sludge I have ever seen. It was like brownish mayo, especially in the way it didn't separate. The system was completely full of this stuff, after two days the radiator was still drizzling out a stream of it, we had to have the radiator rodded out. The pan of drained goo has been sitting two weeks and has not separated, so I wouldn't think there is a pocket of oil left in there.
The engine does have fresh clean oil in it, and the oil that drained out on top of the flushed material was a nice honey color.