Many high performance refrigeration systems use synthetic oils.
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Refrigeration Compressor Oils
Refrigeration compressor oils must be able to perform at low temperatures and while mineral oils are used at higher refrigeration temperatures, some equipment requires synthetic oils. These are usually polyalphaolefin or alkylbenzene-based.
The reason for this is that mineral oils tend to thicken in a non-linear manner at reduced temperatures, with the viscosity increasing rapidly over a few degrees. The same does not apply to the synthetic oils employed, as they are pure substances and not the mixture of different melting point hydrocarbons that make up a typical mineral oil.