This paper reviews recent developments regarding the influence of oil on several thermophysical phenomena in reciprocating compressors. Besides the more essential role of lubrication, the oil is responsible for several tasks in the compressor, from cooling to keeping a low system pressure while the compressor is idle. Thermodynamics dictates the number of phases in equilibrium and the amount of dissolved refrigerant the oil can retain in a liquid phase under a particular condition. Hence, when full miscibility exists, the viscosity of the lubricant mixture is directly affected by the refrigerant solubility in the oil. Heat transfer is crucial for keeping a low temperature in the compressor because thermodynamic losses increase with increasing gas temperature. Fluid mechanics is essential to guarantee that oil is delivered to the bearings and that lubrication is performed efficiently under any operating condition.