The invention relates to a monitoring system (SY1) for monitoring an engine (6) comprising: a sensor unit (U1) connected via an inlet line (12) to an oil circuit (C1) of the engine, the sensor unit comprising one or more sensors (SN) for measuring first parameters representative of the quality of an oil (OL) taken from the oil circuit; an analysis unit (U2) for establishing, based on the first parameters (PR1) or second parameters (PR2) obtained from the first parameters, whether at least one predefined condition (CN) is met, by comparing the first or second parameters with at least one threshold value; and a detection unit (U3) configured to detect a deterioration (EVT) of the quality of the oil representative of an abnormal state of the engine, if at least one predefined condition (CN) is met. The invention also relates to a corresponding method.