There is a little oil in my reservoir. When I look at old post they talk about the oil cooler gasket, but they are all about the X164. Is this also a likely suspect on the X166? Where is the oil cooler, it's not in the same place as the X164?

Can you detect the type of oil - transmission or engine? Does the smell give any hint?

What is the year and VIN for the vehicle?

My first guess is engine oil. When I get some on a paper towel it looks black. Vehicle is a 2013 GL450.
4JGDF7CE1DA232554

Both oils look black. Oil cooler is on the front engine cover underneath all pooleys. Could be gasket, could be oil cooler itself. Transmission cooler is on the left side of radiator, just a pipe inside of it, small chance of leak.

On the right lower next to AC compressor

So much stuff in there I can't tell what's going on. Can't find any past posts about it either. Does someone have the process needed to get the job done?

Item 13, installed near the number 3 in the diagram. A gasket is also needed. Check your VIN at mbparts.mbusa.com

I don't have the WIS for this, but if it was my vehicle I would remove the front underbelly pan and spend time looking at it with a bright LED flashlight. From arsupisemnet's photo it looks like 4 screws to remove the cooler, and the coolant hoses which should be attached by squeeze spring clamps. Need the torque spec for the screws, don't want to strip the threads in the front cover.

I would drain the oil and coolant before removing the cooler. Coolant fill WIS is probably on this site somewhere for M278.

I actually saw it, it's really buried. I think I can work my way into it. Thanks