Doesn't sound good. Can you give out some history of the 5320.. Things that have been done, and that have not been done for maintenance.
Also hours on the engine, location in your profile helps, any change in engine power over time?
My first guess would be possible engine oil cooler leak. Could be a head gasket or cylinder liner also. Is there any sign of coolant in the oil? I would recommend removing the oil cooler and having it pressure tested.