You should change the air dryer filter since it will plug up and pass oil into the rest of the air system (oil in the system can cause the rubber seals in the brake valves to swell and lead to brake valve leakage/failure).
There is a good video series on the CAT engines by "Adept Ape" - here is the one on air compressors
https://www.youtube.com/watch?v=846bVMd_J4QHe mentions that there is an updated air compressor (if you don't already have the updated compressor - just be aware of it in case you need a new compressor). The old style is a twin-piston (which he says is more prone to failure), the new style is a Bendix single-piston compressor.
The three common causes of oil in the air system is 1) compressor rings worn/leaking, and 2) restriction in the air inlet (clogged air filter and/or restriction in air inlet hose to compressor), and 3) excessive compressor cycling (leaking air system, faulty governor, etc.).
The easiest/best thing to do now, is change the air dryer cartridge and check the air inlet hose and filter for restriction.