Abstract The solubility of R290 in the oil and the R290/oil mixture viscosity in the oil sump of rotary compressors used in air conditioner are important parameters affecting the compressor reliability, system performance and R290 charged mass in system. In the study, a set of experiments were carried out in both compressor performance test platform and system psychrometric chambers for an R290 rotary compressor with mineral oil having high viscosity. The dynamic viscosity and temperature of R290/oil mixture at some points within the oil sump of compressor, besides the temperature on shell outside corresponding oil sump and the performance parameters of compressor or air conditioner were measured under different cooling and heating conditions. The variation ranges of oil sump mixture viscosity and R290 solubility were given. The saturated solution temperature and superheat degree of oil sump mixture were proposed and calculated. The effect of compressor suction superheat on the oil sump superheat and its effect on solubility and viscosity was analyzed. A greater suction superheat degree was suggested for an R290 air conditioner with mineral oil in terms of compressor reliability because of the low discharge temperature of R290 compressor.