Many individuals have ask the question over the years " Why does there seem to be oil leaking around cylinders and gaskets on military compressors??"
All of the military compressors that are used in aircraft applications have a common design issue that is not present with most all commercial compressors (ie. Davey, Coltri, Mako, Bauer) -- That is, they are designed to operate in all positions -- upside down, vertical, sideways, etc. The oil pick up is free floating so if the aircraft is upside down the oil pickup is still immersed in the oil. Also they have an oil pump that keeps pressure in the crankshaft to the pistons, so, no matter what the aircraft orientation, the compressor does not experience oil starvation.
The other factor contributing to oil seep in military aircraft compressors is that the oil reservoir and the crankcase are under pressure all the time -- UNLIKE commercial compressors that have a large air breather on the crankcase so NO pressure remains in the crankcase -- in fact the breather usually vents into the intake of the 1st stage so no pressure occurs inside the crankcase to force oil against the gaskets and any oil vapor is never seen by the customer and eventually is captured by the moisture trap/filter.
In forty years I have never determined a solution for this oil seep and in the last 10 years is has become more of an issue since the oils have much higher operating temperatures and extreme pressure additives
I hope this explanation may help answer some questions regarding oil issues.
Just wipe it off regularly and don't be alarmed UNLESS you have an major leak because an oil seal failed.
Jim Shelden