Your engine is turbocharged and aftercooled, and after coolers almost always have a small drain hole in the aftercooler housing to allow condensation
to drain from the housing instead of going into the engine. Typically there is a small hole drilled, and then a roll pin is installed in the hole.
The reason is that the intake air is heated by the turbo compressor
and then cooled back down somewhat by the aftercooler. This cooling
causes moisture to condense on the fins and tubes of the aftercooler element, and then the moisture drops to the bottom of the housing.
It looks like oil
because there is a bit of oil mixed with the water
. The oil works its way past the turbo seals
, particularly if the engine is run at low load for long periods of time. Low load means the turbo isn't working hard and there is no boost pressure in the inlet tract, but instead there is some slight vacuum present. This vacuum pulls oil past the seals
. If you run the engine harder so that some boost pressure develops the amount of oil should be lessened.