A process is disclosed for improving the Viscosity Index of a hydrocarbon lubricating oil comprising the steps of providing a flow reactor having separate first and second inlet ports for the separate co-injection of lubricating oil and an organic peroxide, charging said lubricating oil and said organic peroxide of said flow reactor through said first and second inlet ports respectively, controlling the relative flowrates of said lubricating oil and organic peroxide reactants together with the total volumetric flowrate through said flow reactor to maintain a flow regime which favors diffusional mixing between said organic peroxide and said lubricating oil, and maintaining said organic peroxide and said lubricating oil under conversion conditions including temperatures of between about 50.degree. and 300.degree. C. and pressure sufficient to maintain said lubricating oil and said organic peroxide substantially in the liquid phase. Controlling the flow regime to favor diffusional rather than convective mixing between the lubricating oil and the organic peroxide has surprisingly been found to markedly enhance Viscosity Index improvement.