Mineral oil (MO) has long been utilized in highvoltage
power transformers, as a cooling and dielectric medium;
because of it is availability as it is extracted from fossil oil.
Recently, Gas-To-Liquid (GTL) has a significant increase in use
due to its superior performance, its purity and chemical
consistency over conventional mineral oils. This paper presents
examination of the temperature effects on mineral (Diala S2)
and gas to liquid (Diala S4) oil physical properties. Fresh oil and
aged oil samples, of the two oil types, have been tested in this
paper. Accelerated thermal aging test have been conducted in
laboratory to simulate the heat stress experienced by
transformers during operation. Three aged oil samples, of the
two types of oil, have been prepared in laboratory (3, 6, and 10
days). Density and viscosity measurements were performed on
these fresh and aged oil samples. The results concluded that
during the initial years, chemical changes cause a minor drop in
the density of the mineral oil. Conversely, GTL oil has superior
qualities since it stays more stable over time. Over time, the
density of both oils exhibits comparable stability, however GTL
oil is favored over mineral oil due to its consistency and stability.
Diala S4 and Diala S₂ oils have higher viscosity, suggesting
superior cooling performance before aging. However, Diala S₂
dropped to 10.649 mm²/s after three years, indicating its longterm
consequences. Diala S4 oil had higher flow efficiency,
enhancing transformer cooling and insulating performance
within thermal aging. This is due to GTL oil, derived from
natural gas, has higher resistance to chemical deterioration,
allowing it to sustain its viscosity stability over extended periods.
These results emphasize the importance of choosing oils,
especially GTL oil type based on their apparent and physical
stability over time.