wozzlegummich

The wife's ML370 has started using water. On top up, I noticed globules of creamy coffee-coloured oil in the coolant reservoir. The car is NOT overheating.

Is there any way for the oil to enter the cooling system apart from the obvious, blown head gaskets or cracks in the head and or block?

Odd Piggy

Blown head gaskets usually cause overheating, excessive coolant system pressure, and even radiator and hose failures. Maybe you are catching the problem before it gets to that point. Do you have the ability to do a cylinder compression check?

I'll bow out if anyone has more information.

wozzlegummich

Yes, but surely if compression was down enough to leak the engine would be misfiring?

Odd Piggy

Mostly, yes. But I have had non-MB engines where the lack of compression early on did not cause noticeable misfire, yet one of these blew radiator hoses off.
Compression check on the driver side will be a bit of a chore because of clearance, but if you have changed spark plugs you know that.
There are some MB engines where the oil galleries are sealed from the water jacket at the water pump by an o-ring that can get brittle and leak. However, I have had the water pump off this exact engine and did not see any way that could happen.
Some reports have also been made about the transmission cooler in the radiator leaking. I don't know if that applies to this vehicle. Mine leaked oil from the valve cover and the rear main seal nearing 200k miles, but oil never entered the cooling system.
Do you have a local indie mechanic who will talk honestly to a do-it-yourselfer? Some will, some won't.
I'm out of suggestions so we'll have to wait to see if any of the former MB mechs chime in.