I have seen this a couple of times.
Varnishing is always a result of too high running temperature. Even if your compressor doesn't trip, it can still cause varnishing of the oil if it runs too long at high temperatures.
For plugging with dirt, check the inlet air. If the air is dusty, try installing some extra air filters. All the dirt in the air that is sucked in by the compressor will eventually end up in the oil, oil filter and oil separator.
There's a lot of info on air compressor troubleshooting here:
http://www.air-compressor-guid...troubleshooting.htmlgood luck!
Pawon