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About ac & compressor oils
Why an AC system needs oil at all
The compressor is a pump with moving parts, and it is lubricated by oil that travels around the system mixed with the refrigerant. That is why the oil type has to match both the compressor and the refrigerant. Get it wrong and the oil either will not circulate or will not mix, and the compressor is the part that pays.
PAG, POE and mineral
PAG oils are used in most modern car AC systems running R-134a or R-1234yf, and they come in ISO grades such as PAG 46, PAG 100 and PAG 150. POE, a polyol ester, is common in electric compressors on hybrids and EVs, because it does not conduct electricity the way PAG can. Mineral oil belongs to older R-12 systems and to many air compressors. Substituting one for another is not a small shortcut; it is the usual cause of an early compressor failure.
Which PAG grade
The compressor maker decides, not the car maker, and the number is often printed on a label on the compressor itself or listed in the service data. Where our guides list a grade for a specific model, treat it as a starting point and confirm it against that label.
How much oil goes in
Capacity depends on the system, and it is split between the compressor, condenser, evaporator and drier. After replacing a part, you add the amount that part holds rather than a full charge. Too much oil coats the inside of the evaporator and coils, cooling drops off, and pressures read high. Too little and the compressor runs dry.
Leaks, smells and noise
Oily residue around a fitting usually means refrigerant is escaping there too, since they travel together. A burnt or oily smell from the vents, weak cooling after a recharge, or a compressor that grumbles under load are all worth diagnosing before adding anything.
Home units and air compressors
Home AC systems are sealed and are not a DIY oil job, but knowing what is inside helps when you talk to a technician. Piston air compressors are different again: most use a non-detergent oil, often ISO 100, changed on an hours schedule.
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