PROBLEM OIL IN WATER BUT NOT IN SUMP WHATS MY PROBLEM THINK I MAY KNOW BUT HOPE NOT ANY GARAGE IN EAST ANGLIA THAT CAN DO JOB NOT AT DAF RATES LAST THROW OF THE DICE REGARDS 
oil cooler, oil cooler is not a big job, its the engine flush to get the oil out of the water system that is the problem, mine had to have a new radiator and its not cheap.
deffo oil cooler m8, happend to my one couple of weeks ago, changed it myself n wasnt that hard to do, i got advice from our local daf garage, took me 3 hours from start too finish and that includes flushing out the system
Possibly could be your oil cooler that’s failed, have heard of a few of these failing on Daf’s. I had a similar problem with mine which was caused by the head gasket failing. although it started loosing water first, i put the sealant in recommended by Daf which cured the problem for about 9 months then it started to loose water again but at the same time i noticed the oil contamination in the coolant system.
definatly the oil cooler mate, we had a scania oil cooler gone and the header tank was full of sludge so would say thats your problem