Almost all refrigeration compressors contain oil, which is necessary to lubricate the compressor's internal bearing surfaces. As part of the normal compression cycle, some oil will be pumped out of the compressor with the refrigerant. Any oil that leaves the compressor must return to it; otherwise enough oil can be pumped out of the compressor to cause internal damage.
As part of any maintenance or service inspection, the oil level of the compressor should be checked. Unfortunately, this is not always possible since not all compressors have an oil level sight glass. An oil level that covers one-fourth to one-half of the sight glass is normally acceptable, but always follow a compressor manufacturer's recommendations for the proper oil level on a particular compressor.