Electrical insulating oil

An electrical insulating oil consisting essentially of (A) 80 - 99 parts by weight of a refined oil (I) containing not more than 0.25 wt. % of sulphur and more than 25 wt. % to not more than 35 wt. % of aromatic compounds, the refined oil being produced by the steps of:

• refining with a solvent capable of selectively dissolving aromatic compounds a distillate contained in a fraction having a boiling range of 230.degree. - 430.degree. C at atmospheric pressure obtained by the distillation of a paraffin or mixed base crude oil at atmospheric pressure or the distillation at a reduced pressure of a bottom oil obtained by the distillation of the crude oil at atmospheric pressure thereby to obtain a raffinate from said distillate,
• hydrofining the raffinate so obtained and
• dewaxing the thus hydrofined raffinate with a solvent
• (B) 1 - 20 parts by weight of an unhydrofined refined oil (II) prepared by treating at least with a solid adsorbent a lubricating oil fraction of a mineral oil having a boiling range of 230.degree. - 460.degree. C at atmospheric pressure obtained from a crude oil thereby to obtain a base oil for the electrical insulating oil, the base oil having a total sulphur content of not more than 0.35 wt. %, and (C) 0.001 - 1.0 part by weight per 100 parts by weight of said base oil, of an essentially amorphous ethylene-propylene copolymer (III) having a weight average molecular weight of 10,000 - 200,000 and a propylene content of 10 - 70 mol %, whereby is obtained the electrical insulating oil having a low pour point as well as excellent oxidation stability, thermal stability, corona resistance and corrosion resistance.