It's a Lubricant. It lubricates parts.
To prevent wear on the compressor.
to lubricate the compressor
mineral oil
The compressor oil
Oil in a AC compressor is there so that the mechanical parts don't over heat from friction
freon and oil
Ice
ACID
freon and oil
Sludge in refrigeration is a combination of water, acid and oil, normally this combination will eat the varnish on the windings of the compressor and short it out.
Floc in refrigeration oil is wax that has separated out from oil containing parathene.
Oil in the compressor may leak into the refrigeration system, this oil must be allowed to settle back into the compressor, or it may cause a breakdown due to system blockage