A diagnostic system for a lubrication circuit of an internal combustion engine of a vehicle. The system includes a viscometer for detecting the viscosity of a lubricating liquid of the lubrication circuit, a temperature sensor for detecting the temperature of the lubricating liquid, and a control unit to acquire the state of the lubricating liquid, given by the viscosity detected for a given lubricating liquid condition, which includes the lubricating liquid temperature and the date of last replacement of the lubricating liquid, and for a given condition of use of the engine, and to assess the state of the lubricating liquid by comparing the detected viscosity of the lubricating liquid with the viscosity reference values stored in the database in the same or similar condition of lubricating liquid temperature, date of last replacement of the lubricating liquid and use of the engine.