This paper is devoted to a comparative analysis of thermal ageing tests of Thermally Upgraded Kraft (TUK) and Nomex-910 papers dipped in palm kernel oil methyl ester (MEPKO), at 110, 130, and 150 °C, for 96 h each. After ageing, paper breakdown voltage (BDV), oils’ dissipation factor, dielectric constant and oxidation stability were determined. Nomex’s BDV was greater than TUK BDV. However, the dissipation factor of the Nomex samples is slightly higher than that of TUK, and the dielectric constant has the opposite behaviour. In addition, the decay content of Nomex/MEPKO was greater than with TUK/MEPKO. These experimental findings indicate that Nomex can be mainly used in high voltage, and TUK in medium voltage power transformers respectively, to allow judicious use of their individual characteristics and money savings.