The oil of a refrigeration compressor is not like engine oil: it circulates in the same circuit as the refrigerant, and it has to stay fluid at −40 °C, not break down at a discharge temperature above 120 °C and be miscible with the refrigerant. It forms an oil film in the bearings and the cylinder, seals the compression clearances and carries heat. A small part of the oil is carried into the system with the discharged gas and has to come back; this is why miscibility with the refrigerant is the first criterion in oil selection. The quantity of oil in the system also matters; too little harms the bearings, too much the evaporator capacity.
There are three main types. Mineral oil (MO) and alkylbenzene (AB): used with chlorinated refrigerants such as R22 and with ammonia; they absorb little moisture. POE (polyolester): the oil of R404A, R134a, R449A, R448A, R407C and HFO blends; today it is the standard of commercial refrigeration. PAG (polyalkylene glycol): found in vehicle air conditioning and in some CO₂ and ammonia systems. Viscosity is given by the ISO VG grade; common values are 32 and 68, and the compressor nameplate tells you the right one.
The weak point of POE is that it attracts moisture. In the open air it absorbs moisture within minutes; moist POE produces acid with heat, and the acid attacks the varnish of the motor winding and copper surfaces. This is why the oil container is opened just before use, leftover oil is not kept, an opened circuit is not left open for longer than 15 minutes and the filter drier is replaced at every intervention. Even a good vacuum cannot fully remove moisture bound to POE; the element that really holds the moisture is the drier.
What to look at on site: the oil level should be between a quarter and three quarters of the sight glass. If the colour of the oil has turned from light yellow to dark brown, or it smells burnt, an acid test is carried out; if the result is bad the oil and the drier are replaced. Oils of different types are not mixed. In a retrofit from R22 to HFC the proportion of old mineral oil is brought below 5%. Foaming oil is the sign that liquid refrigerant is returning to the crankcase; the crankcase heater and the superheat are checked.