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the O rings around the cylinder liners would be the first suspect. If you drop the oil pan while it sat for a week, you should be able to see the effects of water seeping: perhaps some surface rust, or white milk which forms when water and oil mix.
You could pressurise the cooling system with an air compressor, just dont go over 1 bar or you'll find a whole bunch of new leaks...
Try the heat exchange unit, if it has one. I have a 7341 super turbo which developed water in the oil and it was not the head gaskets. The heat exchanger cools the engine oil by passing water from rad through the exchanger, the plates in the exchanger can leak water into the oil.