This article introduces a proposal for a condition determination model based on an analysis of the state of the engine lubricants in emergency vehicles (ambulances). This work monitors the evolution of engine oil degradation in order to assess the potential impact of a Condition Based Maintenance (CBM) policy on the emergency vehicle fleets of the National Institute of Medical Emergency (INEM) and INEM ambulances belonging to Fire Brigades. Through the analysis of the lubricants in use, relevant information can be gathered on the operating conditions of the engines emergency vehicles (ambulances), which are essential for optimizing their operation and maintenance. The aim of this study is to determine the time intervals corresponding to the required level of reliability and, consequently, the most appropriate intervals for planned maintenance interventions. The aim is to increase the availability of this type of vehicle, which is critical and often in short supply to respond to the large number of daily demands. The analysis of a lubricant includes the determination of various physical-chemical parameters and an analysis of wear metals. This monitoring of lubricant oil degradation also increases the safety of emergency vehicles, reduces costs and improves their reliability and efficiency. The model for monitoring the degradation of diesel engine oils applied to the INEM Fleet and Fire Brigades, which is the subject of this study, has the potential to be generalized to any Civil Protection organization aiming to implement a preventive maintenance policy with an emphasis on condition monitoring.