A method for introducing a refrigerant and lubricant into a vapor-compression refrigeration device system comprising: a. providing a vapor-compression refrigeration device comprising a heat transfer circuit, a compressor having an inlet side and an outlet side, and refrigerant and lubricant reservoir, wherein said reservoir is in fluid communication with the inlet side of the compressor and with said heat transfer circuit, and said heat transfer circuit is in fluid communication with said outlet side of the compressor; b. determining a lower operating temperature range of a vapor-compression refrigeration device; c. determining an upper operating temperature range of the vapor-compression refrigeration device; d. selecting a refrigerant comprising at least one C 3 to C 4 fluoroalkene at a first concentration and selecting a lubricant comprising at least one polyol ester or polyalkylene glycol at a second concentration to produce a fluid system having a refrigerant-rich phase and a lubricant-rich phase at a first temperature within said lower operating temperature range and at a second temperature within said upper operating temperature range provided that said second temperature is higher than said first temperature, wherein the refrigerant-rich phase is denser relative to the lubricant-rich phase at said first temperature, wherein the lubricant-rich phase is denser relative to the refrigerant-rich phase at said second temperature, and wherein the difference in density between the refrigerant-rich phase and the lubricant-rich phase is 20% or less; and e. introducing said refrigerant and lubricant into the vapor-compression refrigeration device. a. providing a vapor-compression refrigeration device comprising a heat transfer circuit, a compressor having an inlet side and an outlet side, and refrigerant and lubricant reservoir, wherein said reservoir is in fluid communication with the inlet side of the compressor and with said heat transfer circuit, and said heat transfer circuit is in fluid communication with said outlet side of the compressor; b. determining a lower operating temperature range of a vapor-compression refrigeration device; c. determining an upper operating temperature range of the vapor-compression refrigeration device; d. selecting a refrigerant comprising at least one C 3 to C 4 fluoroalkene at a first concentration and selecting a lubricant comprising at least one polyol ester or polyalkylene glycol at a second concentration to produce a fluid system having a refrigerant-rich phase and a lubricant-rich phase at a first temperature within said lower operating temperature range and at a second temperature within said upper operating temperature range provided that said second temperature is higher than said first temperature, wherein the refrigerant-rich phase is denser relative to the lubricant-rich phase at said first temperature, wherein the lubricant-rich phase is denser relative to the refrigerant-rich phase at said second temperature, and wherein the difference in density between the refrigerant-rich phase and the lubricant-rich phase is 20% or less; and e. introducing said refrigerant and lubricant into the vapor-compression refrigeration device. <EOS>