Genadi
the first question is, does it lose water? If it does, and you have water in the sump ("Karter"), the
chances are this is internal, not external (some times you can get water in the sump if the weather is very humid, but it should not be much, or someone pressure washed the engine and left an oil filler cap open, or an external hosepipe is leaking straight into the sump, but I can't see how that's possible).
If that's internal, it can be one of the next: (i) your head gasket (Packung rosh) is dead - this means head off, a bit of work, but not a disaster (can do in place, no need to take engine out. On a diesel with an iron head like the 400 the head should not be warped either (=$$)) (ii) you have a cracked block - this will be the killer, because you have to take the engine out, completely dismantle and do a pressure check to find out where it's coming from... And then the question is, can you weld the crack... $$$$$$$$$$$:shock:. Oh yes, if he tells you you can cure it with Bars Leaks or any such sealant, well, if it's a packung rosh, this is a bodge (camoflage, "iltur") and is a big gamble - even if it works for some time, so I would only do this as an emergency to get you home.
Oil in the water sounds like packung problem to me
Hope this helps, T