We have modified the mineral oil used in transformers by dispersing 1-wt.% metal oxide nanostructures (commercially available [Formula: see text], ZnO and [Formula: see text] spherical structures and ZnO rod-shaped structures synthesized by arc discharge) into the oil through ball milling without surfactant. A good dispersion that lasted for at least 24 h was obtained for all nanofluids, however sedimentation was discovered by 72 h after ball milling. All nanofluids with different nanostructures exhibited enhanced thermal conductivity compared with the raw transformer oil. The nanofluid with ZnO nanoparticles showed better thermal conductivity than the nanofluids with [Formula: see text] and [Formula: see text] nanoparticles. The nanofluid with elongated ZnO nanoparticles (nanorods) synthesized by arc discharge showed the best thermal conduction among all the nanofluids studied in this work over the whole measurement period. The enhanced thermal conductivity of the nanofluid with elongated nanostructure is considered to be due to the rod-shaped nanostructure creating heat flow paths with lower thermal resistance. The arc discharge provides a cost-effective and scalable method to fabricate metal oxide nanostructures for potential nanofluid applications.