I've seen some issues with moisture contamination while the sample was in transit. If you're going to pull oil samples on these machines, you need to use a glass bottle with a gasketed cap. There are no drier cores to be replaced on any RTAC manufactured before 2011, either. Because of POE's affinity for moisture, the oil is the drier in the system. I have a document somewhere with the numbers. I'll try and find it for you. Paradoxically, R-134A systems are more forgiving than R-22 systems where moisture is concerned.
If the circuit went flat and was allowed to sit open for more than a day or so, I would have recommended an oil change on this circuit.