In this paper, a combination of mineral oil and synthetic ester oil in a standard mass ratio is used as an alternative liquid dielectric for power transformers instead of traditional mineral oil. In these mixed oils, the two alternative solid dielectrics of transformers, namely thermally upgraded Kraft and Nomex-910, are impregnated. An accelerated aging test is performed on these samples at different temperatures ranging from 100 to 180 °C. Further, ultraviolet spectroscopy test is performed in order to determine the thermal degradation as well as oxidation rate of these samples. It has been observed from the test results that Nomex impregnated mixed oil samples have lower oxidation rate as compared to that of thermally upgraded mixed oil samples. Therefore, Nomex impregnated transformers have lower oxidation rate and longer service life as compared to TUK impregnated transformers. The present experimental study is very beneficial to the transformer manufacturing companies as well as utilities.