Is this diesel or oil in the coolant

[Chanoo]

Hi folks,
Got a 2002 defender TD5, this issue with fuel or oil or both in the coolant (water ATM).
The pics below are two containers, the container on the left is water and diesel (done just for comparison) and the larger container on the right is what came out of the expansion tank. I want your opinion on what is the black dark substance.
Is it diesel or oil or a mix of both?
What is the likely source?
Thanks https://www.aulro.com/afvb/images/imported/2018/02/687.jpghttps://www.aulro.com/afvb/images/imported/2018/02/688.jpg

[Tombie]

Looks like oil.
The oil cooler has likely failed internally on the side of the motor

[djam1]

I had this happen due to the wrong coolant being used.
The other possibility is that the O rings have failed on the unit.
Its behind the turbo under the alloy cover hopefully it wont cost you a head as well, it did in my case.
You will need to replace all hoses as well.