Abstract: An electrical insulating oil composition consisting essentially of 50-85 parts by weight of a paraffinic or mixed base crude oil-derived electrical insulating oil, 50-15 parts by weight of a naphthenic base crude oil-derived electrical insulating oil, and 0.001-1.0% by weight of a hydrocarbon-derived pour point depressant, based on the total weight of said two oils.

Abstract: An electrical insulating oil composition consisting essentially of (I) 50-95 parts by weight of a mineral oil-derived electrical insulating oil, (II) 5-50 parts by weight of at least one oil selected from diarylalkanes, polybutene, an oil having a specified refractive index and a hydrocarbon oil-derived hydrofined oil, and (III) 0.001-1.0% by weight of a hydrocarbon-derived pour point depressant, based on the total weight of the oils (I) and (II).

Abstract: An electrical insulating oil having excellent oxidation stability, thermal stability, corona resistance and corrosion resistance and, if desired, low-temperature properties, which consist essentially of a blend of a refined oil (I) derived from a paraffin or mixed base crude oil, a refined oil (II) prepared from a lubricating oil fraction of a mineral oil and, if desired, an amorphous ethylene-propylene copolymer (III). The oils (I) and (II) may be blended together in ratios by weight of 80- 99:1- 20. In one embodiment, the copolymer may be added to the blend of the oils (I) and (II) in ratios by weight of 0.001- 1.0 to 100 thereby to obtain an electrical insulating oil having excellent low-temperature properties in addition to the above-mentioned excellent properties.

Abstract: An electrical insulating oil composition having satisfactory oxidation stability, electrical properties and low-temperature performances, consisting essentially of (I) an electrical insulating oil having a sulphur content of 0.1-0.35 wt.% prepared from a paraffin or mixed base crude oil and (II) 0.001-1.0% by weight, based on said insulating oil, of an essentially amorphous ethylene-propylene copolymer having an average molecular weight of 10,000-200,000 and a propylene content of 10-70 mol%.

Abstract: An electrical insulating oil consisting essentially of (1) a mineral oil, as the base oil for the product oil, containing not more than 0.35 wt.% of sulphur and being prepared by refining a distillate containing at least 80 wt.% of a fraction boiling at 230.degree.-430.degree. C at atmospheric pressure, the distillate being obtained by the distillation of paraffin or mixed base crude oils, (2) at least one arylalkane and, if desired, (3) a hydrocarbon-derived pour point depressant.

Abstract: An electrical insulating oil having excellent thermal stability and low-temperature properties, which consists essentially of (A) 80-99 parts by weight of refined oil having a low sulphur content and considerable aromatic content and being prepared from a distillate contained in a fraction boiling between 230.degree.-430.degree. C of a paraffin or mixed base crude oil origin, (B) 1-20 parts by weight of a refined oil prepared from a lubricating oil fraction of a mineral oil boiling between 230.degree.-460.degree. C of any crude oil origin and, if desired, (C) 0.001-1.0 part by weight of an essentially ethylene-propylene copolymer as the additive.

Abstract: An electrical insulating oil having excellent oxidation stability, thermal stability, corona resistance, corrosion resistance and, if desired, a particularly low pour point, which consists essentially of a blend of a refined oil (I) derived from a paraffin or mixed base crude oil, a refined oil (II) prepared from a lubricating oil fraction of a mineral oil, at least one arylalkane (III) such as an alkylbenzene, and, if desired, an essentially amorphous ethylene-propylene copolymer (IV).

Abstract: This invention provides a method for the preparation of insulating oil having good oxidation stability, electric characteristics and resistance to copper corrosion which comprises subjecting a distillate (stock) within a temperature range from 250.degree. to 400.degree. C. in terms of the boiling point at atmospheric pressure to solvent refining to a desulfurization from 30 to 75% by weight to give a raffinate, said distillate having been obtained either by distillation at atmospheric pressure of a paraffin- or mixture-base crude oil and/or by distillation under reduced pressure of a residual oil from the atmospheric pressure distillation, subjecting said raffinate to hydrogenating refining to a desulfurization from 40 to 90% by weight and subjecting the refined product to solvent dewaxing, and if necessary, subsequently subjecting the dewaxed product to clay treatment to a sulfur content of the final product from 0.1 to 0.35% by weight.