Mack Scania

Oil cooler is probably the culprit

 

Old Scanias especially the v8s often leaked through the oilcooler, that's the engine you're talking about?

 

On most engines if there is a leak in the water / oil heat exchanger you will get oil in the water as well.
The reason is that while the motor is running the oil pressure is higher then the pressure in the water.
You then get oil to water transfer.
On shut off the oil pressure drops to zero while the water pressure remains elevated while the water temperature is above ambient.
You then get water to oil transfer.

You can get water/coolant in your oil from a leaky seal on a wet liner type engine.
Very common.

You can get water/ coolant in the oil if there is a leak in a head gasket from a water port through to a oil return.(Especially common if the engine got hot)
This can also happen in a water cooled compressor.

There is also a possibility of a small crack in the head causing this, often as a result of an over torqued injector.