A petroleum oil of improved stability (e.g., as an electrical insulation medium), can be prepared by a process comprising blending 25-98 parts (preferably 50-95) of a severely hydrorefined oil (preferably, a hydrorefined naphthenic or mildly aromatic oil having a VGC from 0.820-0.940, a viscosity from 30 to 200 SUS at 100*F. and an ultraviolet absorptivity (UVA) of less than 0.04 in the 335 millimicron region (i.e., 335 UVA)) with from 75-2 parts (preferably 50-5) by weight of unhydrogenated naphthenic distillate (which preferably is free of naphthenic acids, and more preferred, has a pour point less than 0*F.), and then extracting the blend with an aromatic selective solvent (e.g., furfural, phenol) to obtain a raffinate oil product. The raffinate can be finished, as by adsorbent contact with attapulgite, acid activated clay or mixtures of such adsorbents. From 0.1-30 weight percent of a cycle oil can be added to the blend before extraction or to the raffinate oil product.