A process oil comprising a 95/5 to 5/95 by weight mixture of: an extract having a DMSO extractable content less than 3 wt % obtained by solvent-extracting an oil obtained by deasphalting a vacuum distillation bottom; and either a mineral oil base having a DMSO extractable content less than 3 wt % which is a raffinate obtained by the solvent refining of a vacuum distillate or a mineral oil base having a DMSO extractable content less than 3 wt % obtained by subjecting a raffinate obtained by the solvent refining of a vacuum distillate to hydrorefining and/or dewaxing. The process oil has a kinematic viscosity at 100.degree. C. of less than 32 mm2/s, % CA of 15 to 30, aniline point of 100.degree. C. or lower, weight change through evaporation of 0.5% or less, and mutagenicity index of less than 1.