Soot-induced thickening can lead to significant increases in lubricant viscosity in modern diesel engines and forms a major part of diesel engine oil specifications. Despite its importance, relatively little is known about the viscosity characteristics of soot containing oils over conditions relevant to a wide range of engine operating conditions. In this paper, a detailed study of the rheological properties of soot loaded oils from Mack T-8 and Cummins M-11 tests are described. The rheological behaviour is highly complex, particularly as oils become increasingly non-Newtonian in character due to flocculation of soot particles. The role of temperature is critical in the development of flocculated structures, and its effect on viscosity. Extensive soot thickening in oils under moderately quiescent conditions can lead to marked yield stresses at low temperatures (below zero °C). The relative performance of different oils varies across the range of temperatures studied (100 to -25°C). Rotational viscometry offers an improved means of assessment of the ability of a lubricant to control viscosity increase using a self-contained measurement.