POE (Polyolester) oil is a synthetic lubricant used in Heat Pump Systems that utilize R410A, R32, or R454B. This oil is highly hygroscopic, meaning it absorbs moisture very quickly if the system is left open to the air. This is why a proper Evacuation Process with a vacuum pump is so critical during installation. The oil must stay clean and dry to lubricate the Inverter Compressor properly over its lifespan. We provide technical details on oil types in the Technical Data for each model. If moisture enters the oil, it can create acid that eats away at the system's internal Parts. Protecting the oil is the best way to ensure your Warranty Policy remains valid. It is the "lifeblood" of your mini-split's mechanical components.