Hello Gudjon
Cool Iceland, huh?
I love forums and the Internet: I get to sit in my living room in New Mexico in North America and discuss Isuzu mechanical issues with people all over the globe.
Anyway, the first thing I would do is make sure that is diesel in the oil and not some other fluid. You will need to ask someone with more diesel engine experience, but if it were a gasoline engine I would smell the engine oil to see if the injectors are leaking fuel into the crank case.I don't know if you can smell diesel in motor oil or not. Another way with gasoline is to fill a thimble and light it, obviously diesel won't ignite, so there must be another way to tell what liquid is being added to the oil.
I hope this helps
Brian Rodgers