The present invention provides methods for selecting refrigerant and lubricant combinations for use in heat transfer cycle systems and provides methods for operating said heat transfer systems. More particularly, the invention provides methods to select lubricant and refrigerant combinations for a heat transfer cycle system wherein at the lower temperatures of the heat transfer cycle the refrigerant and lubricant are miscible and at the upper temperatures of the heat transfer cycle the refrigerant and lubricant are phase separated and such that the density phase inversion temperature of the combination is below the upper operating temperature of the heat transfer cycle.