Power losses accompany each electrical device during its work. The proper power losses dissipation to the ambient allows avoiding the device overheating and its operation failure. One of the most effective techniques for the cooling of internal parts of the electric devices, such as transformers, is the implementation of a dielectric coolant within the device casing. Nowadays, the most popular coolants in the market are based on mineral components. However, mineral oil is not neutral for the environment, especially in a case of coolant leakage or its disposal after the device lifetime. Therefore, more environmentally friendly coolants belong to the liquid group of biodegradable oils where the most common representative is ester oil. The presented study focuses on heat transfer modelling in the 315 kVA distribution transformer using coupled models. Electromagnetic model of the transformer based on the Finite Element Method allowed to estimate the transformer power losses. The power losses were interpolated into the Computational Fluid Dynamics (CFD) model as volumetric heat sources. The CFD model was used for heat and fluid flow simulations. The calculations were conducted for mineral and ester oils with different external conditions representing variable climate conditions such as northern Argentina and Poland. The ester oil properties are characterized by higher viscosity and higher thermal conductivity than mineral oil. The hotspot temperature results obtained for the ester oil scenario were up to 3 K higher than for mineral oil case. The temperature range on the external surface of the fins was between 57 and 70 ℃ for Argentinian climate conditions. The numerical results showed a satisfactory agreement with the measured values.