You keep saying that. I agree with the oil sampling just to get an idea of overall engine health. I have never seen freshwater oil mix turn any form of yellow though. Out of 25 years working on engines of all kinds, it's always looked like milk or a light grayish color. If I'm not mistaken, Lou_tribal has said that his engine uses coolant, not raw water, and his coolers are raw water cooled. I may be understanding that wrong. I have never seen brackish or salt water in oil, it may be yellow. I don't think that is the case here. I am in agreement with everyone else, it's probably an internal oil cooler leak somewhere. With Lou stating that there's not been an overheat situation, and the engine is running a touch cold, I doubt it to be an engine issue. I feel that he has a good handle on things and we should wait to hear back from him before making more suggestions.