The compressor’s efficiency strongly affects the efficiency of heat pumps. It is strongly influenced by the fluids in the compressor, i.e., the refrigerant and the lubricant. While the choice of refrigerant has been extensively researched for decades, the choice of lubricant has until now mostly been based on the ability to lubricate the compressor sufficiently. However, lubricant and refrigerant in the compressor interact with each other, and some of the refrigerant dissolves in the lubricant. The dissolved refrigerant changes the lubricant’s effective fluid properties, such as the viscosity and heat capacity. This has impacts on the operation and efficiency of the compressor. The viscosity influences fluid friction losses, with higher viscosities leading to higher friction lo ...