Soot in a lubricated diesel engine is effectively dispersed without adversely affecting the viscosity of the lubricant by using a particular lubricant. The lubricant utilized comprises a lubricant base stock (e.g. more than 75% by weight), a dispersant (e.g. from 0.2-less than 4% from a detergent inhibitor (D1) package), and a functionalized viscosity index improver (e.g. from 0.1-2.5% by solids weight). The functionalized viscosity index improver is a highly functionalized graft copolymer reaction product of an oxygen, a nitrogen, or an oxygen and nitrogen containing, ethylenically unsaturated, aliphatic or aromatic monomer having from 2 to about 50 carbon atoms grafted onto a polyolefin copolymer. Also, 0-2.5% by solids weight of another viscosity index improver besides the highly functionalized graft copolymer reaction product may be added, as well as other conventional additives.