A monitoring system for monitoring an engine comprising: - a sensor unit connected to an inlet line and an outlet line, the inlet line includes a hydraulic pipe connector set up to connect to one of the engine's oil circuits, This sensor unit has at least one sensor set up to measure first parameters representing the quality of an engine lubricating oil taken from the oil circuit via the inlet line, the outlet line is adapted to discharge the oil from the sensor unit into the oil circuit; - an analysis unit configured to determine, from the first parameters or second parameters obtained by processing the first parameters, whether at least one predefined condition is met, by comparing these first or second parameters with at least one threshold value; and - a detection unit configured to detect a degradation of the oil quality representative of an abnormal state of the engine, if at least one predefined condition is met; wherein the processing performed by the analysis unit performs at least one of the following: - determining, as a second parameter, a corrected electrical conductivity from the electrical conductivity measured by at least one sensor by applying a first final polynomial function. The first final polynomial function defines this corrected electrical conductivity from the electrical conductivity, the temperature pressure and relative humidity measured by at least one sensor, this first final polynomial function is weighted by first coefficients (CFCE) associated respectively with the temperature, pressure and relative humidity to compensate at least in part for the respective influence of the temperature, pressure and relative humidity on the electrical conductivity measured by at least one sensor; and - determining, as a second parameter, a corrected dielectric constant from the dielectric constant measured by at least one sensor by applying a second final polynomial function. The second final polynomial function defines the corrected dielectric constant from the dielectric constant, temperature, pressure and relative humidity measured by at least one sensor, this second final polynomial function is weighted by second coefficients (CFCD) associated respectively with the temperature, the pressure and the relative humidity in order to compensate at least in part for the respective influence of the temperature, the pressure and the relative humidity on the dielectric constant measured by at least one sensor; wherein the analysis module is configured to compare at least one of the corrected electrical conductivity and the corrected dielectric constant with a respective threshold to determine whether at least one predefined condition is met. - a sensor unit connected to an inlet line and an outlet line, the inlet line includes a hydraulic pipe connector set up to connect to one of the engine's oil circuits, This sensor unit has at least one sensor set up to measure first parameters representing the quality of an engine lubricating oil taken from the oil circuit via the inlet line, the outlet line is adapted to discharge the oil from the sensor unit into the oil circuit; - an analysis unit configured to determine, from the first parameters or second parameters obtained by processing the first parameters, whether at least one predefined condition is met, by comparing these first or second parameters with at least one threshold value; and - a detection unit configured to detect a degradation of the oil quality representative of an abnormal state of the engine, if at least one predefined condition is met; - determining, as a second parameter, a corrected electrical conductivity from the electrical conductivity measured by at least one sensor by applying a first final polynomial function. The first final polynomial function defines this corrected electrical conductivity from the electrical conductivity, the temperature pressure and relative humidity measured by at least one sensor, this first final polynomial function is weighted by first coefficients (CFCE) associated respectively with the temperature, pressure and relative humidity to compensate at least in part for the respective influence of the temperature, pressure and relative humidity on the electrical conductivity measured by at least one sensor; and - determining, as a second parameter, a corrected dielectric constant from the dielectric constant measured by at least one sensor by applying a second final polynomial function. The second final polynomial function defines the corrected dielectric constant from the dielectric constant, temperature, pressure and relative humidity measured by at least one sensor, this second final polynomial function is weighted by second coefficients (CFCD) associated respectively with the temperature, the pressure and the relative humidity in order to compensate at least in part for the respective influence of the temperature, the pressure and the relative humidity on the dielectric constant measured by at least one sensor;