Used Diesel oil viscosity is commonly measured by the kinematic method as outlined in the Mack T-8 used diesel soot procedure (ASTM D 5967 for used oil from Mack T-8 engine test, and CEC L-83-A-97 for used oil from PSA XUD11 ATE engine test). The limitations of the kinematic viscosity measurement for used oils heavily loaded with carbonaceous matters, have led to a new proposed method which may be better suited for these oils.This paper describes an investigation about the rheology of Diesel oils heavily loaded with carbonaceous matters. The kinematic viscosity result (measured with only the first sphere) was presented as a function of shear rate (imposed by differents kinematic viscometers) and demonstrated that the behaviour of these oils is not newtonian for low shear rates.With a dynamic oscillation rheometer (cone and plate) the study noticed that the oil can have a thixotropic behaviour and the thixotropic effect becomes more important upon the viscosity measurement when the oil is more loaded with soot.With the requirement to measure the viscosity at low shear rate, the Brookfield viscometer was explored. Various investigations will be discussed along with considerations for preliminary preparation, test temperature, geometry and speed of the mobile (rotor), definition of the test recipient (stator), time effects.The test method and round-robin tests are also presented. Results include data from a number of different oils with high, medium and low thickening and different soot levels. The accuracy of the method was estimated, and the repeatability and the reproductibility are the same for fresh and used oils. The results of the round robin show that DP/Δ estimated is less than 1 wich indicates the method is absolutely able to discriminate ‘passing’ and ‘failing’ oils. This paper presents a first step in the validation of the Brookfield measurement opportunity.