“Oil thickening” due to soot has long been recognized as a potential problem in diesel engine lubrication. Soot-related viscosity increase in used crankcase oils is remarkable in that relatively small concentrations of soot produce large viscosity increases. Classical studies of suspensions of rigid nonagglomerating spheres in liquids suggest that the viscosity increase produced by 6% (by weight) soot in an oil should be less than 10%. In fact, the viscosity increase in such a mixture may be as much as several hundred percent. This discrepancy implies that the effective volume occupied by soot particles suspended in oil is very large. Past theoretical and experimental work on the rheology of particle suspensions provides a method of quantitatively determining the effective volumes (“voluminosities”) of soots in oil suspensions. Three different used oils were characterized by controlled shear rate rheometry. Quantitative voluminosities permit the reliable companion of the efficacies of different oils to control soot-related viscosity increase. Mechanisms are speculated upon by which soots exhibit very high effective volumes in suspensions.