I'm surprised that your heat exchangers are freshwater cooled, as on my 4-108, (Westerbeke 40-108), all accessories are raw-water cooled, with only the block being freshwater cooled; (but there are many possibilities in this area).
For $20 you can get a lube-oil sample kit from an auto parts
store. Get a sample of the oil in the engine sump. You will then find out what it really has in it, as opposed to speculating. Fuel oil, (diesel), dilution is a common problem with marine
diesels, and this is what you will probably see. Diesel
oil dilution of the sump will not cause smoke, it will decrease the viscosity of the lube oil. A slight dilution is not much of a problem, and will clean the inside of the engine. I've done this intentionally before an oil change
. A larger dilution can be serious, raising the acidity of the oil and decreasing the viscosity. In the Merchant Marine
of thumb is to make sure to change-out the lube oil at no greater than a 10% dilution.
Another possibility is that the sump can go up and down if the lube oil has drained out of the oil filter
. I always run my engine for a few minutes, (to fill the filter and the oil gallery), then wait a few minutes, (for the cold oil on various surfaces to drip back to the sump), before checking the oil. This way I get consistent results.
I am used to fuel oil dilution of the lube oil coming from dribbling injectors, but, it sounds like it could come from the lift
pump or injection pump too, (as mentioned above).
If the dual cooler pictured above is similar to the one on your engine, there is no way that the oil could have migrated from the transmission
to the engine sump, through the coolers. Though it could get into the cooling
medium. If that is freshwater it would show in your antifreeze
, if raw-water, it would go overboard
, (assuming the oil-pressure is higher than the water pressure, otherwise you might get water in your oil).