This study investigates the potential of a soybean oil–sunflower oil blend as an environmentally friendly alternative transformer insulating liquid with the addition of Diphenylamine (DPA). The mixtures were prepared in ratios of 25:75, 50:50, and 75:25, with DPA concentrations of 0, 0.5, 1, and 1.5 wt%. Viscosity testing was conducted using a Zahn Cup type 2 based on the Standard Test Method…