As a general rule, ester oils are only currently used with R12 or retrofitted AC systems. PAG, in various viscosities, is the recommended refrigerant oil for most R134a vehicles since the mid-nineties. Consult manufacturer service guides for specific vehicles, especially imports and hybrids, as some compressor manufacturers have very specific requirements. Many companies offer PAG in a "universal" form, or in viscosities of 46, 100, and 150.
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