As carbon emission cuts in buildings grow more urgent, heat pumps provide a promising path to sustainable heating. Simulations are crucial for optimizing operation and refrigerant charge, especially with R-290 (propane) gaining traction in European installations. This paper examines the impact of the refrigerant-oil solubility on the system operation. Any refrigerant dissolved in the compressor oil no longer participates as a working fluid, distorting simulation-based charge estimations. The amount of refrigerant absorbed depends on both the oil-refrigerant combination and the system’s operating conditions. A heat pump model using R-290 was developed in Dymola/Modelica to investigate this effect. A semiempirical equation of state was used to determine the amount of refrigerant dissolved. The model was applied to different operating points and oils. Results show that the refrigerant solubility in oil is crucial for accurate system behavior prediction. Neglecting it can lead to misjudgements regarding refrigerant charge and operating strategies.