A lubricating oil reservoir has an open end closed by a cap removably coupled to the open end. The cap is formed with a sensor located in the lubricating oil reservoir. The sensor is for sensing the presence of lubricating oil that fills the lubricating oil reservoir to a recommended oil level for providing machine lubrication, and the non-presence of lubricating oil that does not fill the lubricating oil reservoir to the recommended oil level, and a signal apparatus is operatively coupled to the sensor for issuing a first stimulus in response to the sensor sensing the presence of lubricating oil for identifying a normal lubricating oil level condition, and for issuing a second different stimulus in response to the sensor sensing the non-presence of lubricating oil for identifying a low oil level condition.