The end of insulation paper will lead to an end of the lifetime of the oil-immersed transformer. It is, however, not possible to change the insulation paper as a function of the oil over the lifetime of the transformer. With the aim of extending the lifetime of an aging transformer, the mineral oil is mixed with an ester and poured into the transformer on-site, in the so-called "retrofilling" process. When the insulation oil of a transformer undergoes retrofilling, an ester is mixed with an aging mineral oil. Then, if a partial discharge occurs inside the transformer after retrofilling, we conduct experiments with the discharge responses under a negative impulse voltage applied in a non-uniform field using blended oils, which are blends of three types of ester with aging mineral oil at the ratio of 8:2 in volume. Comparing the discharge propagation velocities in all the sample oils, we measure a primarily constant value (1.0 to 2.0 mm/μs), independent of the applied voltage.