Monitoring and testing of subsurface equipment is crucial when stepping up artificial lift efficiency. Oil production using electrical submersible pumps (ESP) in RN-Purneftegas was initially complicated by a strong gas influence. The main method to increase ESP performance in wells with a high GOR is using rotary gas separators. Subsurface equipment adjustment for high GOR conditions, including gas separator calibration, is of primary importance, due to a strong influence of the ESP design on well performance. To perform the corresponding calculations, appropriate data is necessary. Currently, there are many tools and measurement devices for oil production monitoring and controlling, however, it is impossible to use its data without a good understanding of all the parts of the "reservoir-well-pump" production chain. The artificial lift team formed under Rosneft's New Technology System program conducted a wide range of field tests. The main goal of such tests was gathering information for parameter analysis and proper timing of ESPs with malfunctions in rotary gas separators. The collected information about real field performance was then used for validation of the accuracy of the lab data for gas separator performance, acquired by Russian State Oil&Gas University. It was concluded that the gas separator performance data obtained in laboratory if combined with correlation for natural separation prediction can be used for total separation efficiency estimation. The test results allowed us to estimate the potential for oil production increase at over 700 tons per day in Purneftegas. Importance of separation efficiency for well performance A number of geological and technical factors such as pump performance acquired in lab tests with a single phase fluid (typically water), or well operation history are used in ESP calibration. However, gas separator performance had not been adequately modeled during previous ESP sizing. The separator was treated as an intake module with constant separation efficiency in most cases.