For long-term operation and sustainability, transformers require reliable and eco-friendly oil insulation. Although mineral oils have been used for over 150 years, their high fire risk, low biodegradability, and reliance on nonrenewable sources present challenges. This study aims to systematically evaluate the impact of aged mineral oil (AMO) contamination and fatty acid (FA) composition on the dielectric and physicochemical performance of synthetic (SEO) and natural ester oils (NEO) in transformer retrofilling applications. The novelty of this study lies in the detailed analysis of how varying AMO levels (7.5% and 9.5%) and FA profiles affect key properties—including breakdown voltage (BDV), moisture content, total acid number (TAN), viscosity, color index, and flash/fire points—under accelerated thermal aging at 135°C for 28 days. The results indicate that 9.5% AMO contamination increases BDV, color index, and TAN more significantly than 7.5%. The moisture content in the SEO remained within the IEC 61099 limits, whereas that in the NEO exceeded 200 ppm after 14 days. SEO with a lower C18:1 content (7.64%) demonstrated superior thermal stability compared to NEO, which exhibited higher oxidation susceptibility due to its higher C18:1 level (26.16%). High C18:1 levels reduced BDV, whereas AMO degraded heavy saturated fatty acids (C20–C24). Additionally, the increase in C12 in SEO reduced viscosity, potentially affecting oil flow. These findings clarify the role of FA composition and AMO contamination in optimizing transformer oil retrofilling strategies and support the future adoption of ester-based insulating liquids for more reliable and sustainable power systems. © 2025 Faculty of Engineering, Universitas Indonesia. All rights reserved.