2009 MaxxForce 7.6l Coolant and Oil Mixing

My question is; what is the best way to diagnose where the coolant and oil are mixing? My goal is to determine if the fix is relatively low cost, or if I am looking at needing a rebuild, in which case I will likely just sell the truck.

Below is some backstory on the situation if you care to read it.
I recently bought this truck and drove it 140 miles home with no issues, oil looked great, no lights, ran great. Truck has 187k on it, so not a spring chicken. Last weekend my neighbor expressed interest in buying it for his tree service. Before starting it, oil still looked great and at first it ran great. They drove it a 1/2 mile, it stalled, they started it again and headed back, the water in oil message came on, and there is now a green tint on the dipstick and oil in the coolant tank. It does still start, I ran it extremely briefly, so it presumably has good compression, but I am certainly not interested in running it more until I solve this.
I've been told these engines have a tendency for the cylinder liner o-rings to fail, which as I understand it requires an in-frame rebuild. I am also under the impression it could be oil cooler, EGR cooler, or corrosion on the plate that the water pump mounts to.
I have visually inspected the oil cooler and saw no signs of leaks.
Attached are a few photos of the situation.
And yes, I have now learned the MaxxForce engines are piles of ####. But, I'm either going to fix the pile or not, just looking for some advice.

Also, @Heavyd because you seem like a knowledgeable person based on you other replies.

 

Thank you very much for the reply. I am planning to replace the oil cooler, flush the cooling system and change the engine oil/filter. Anything else I should change as a result of this issue?

Just to clarify, there is enough coolant making it into the oil that the water in oil message came on, and it looks like there could be a little on the dipstick. Would this still align with symptoms of a bad oil cooler? I haven't noticed any big increase in volume of liquid on the dipstick so it doesn't look like a lot of coolant is making it into the crankcase.

Also, is there any way to pressure test the existing oil cooler or otherwise definitively say it is bad before replacing?

 

The OIL/WATER light is what we saw - I took it to mean water in oil - thank you for the clarification. I will call around to the local dealer and some diesel shops to see if they have test plates.

Any recommendation on the best way to flush the cooling system so as to remove as much oil as possible?

 

@Heavyd @Star Rider Thank you both for the guidance.

The truck has been sitting for a couple weeks now. Today I dropped some engine oil and there was no water at all. Oil looks good and does not feel or smell unusual; no water or milky color. I did notice a very slight green hue, would this be indicative of coolant? Photos of the oil are attached.

Since the oil appears good, should I bother doing an oil change after replacing the oil cooler, or just leave it? I also ordered a blackstone test kit.

Is there anything else you would do for diagnosis before replacing the oil cooler and gaskets? Any other helpful tips you can offer on that process are hugely appreciated. I did find this guy's video which will be useful.

Thanks!

 

Thank you, will do. Thanks for the heads up on the bolts. Would it be possible to procure a copy of that manual from you?