When comparing fluids, pay particular attention to the viscosity of the retrofill fluid. As the transformer was designed to be cooled with conventional transformer oil, it will run warmer with a thicker fire resistant fluid. Choosing a fluid with the lowest viscosity possible will minimize this problem. Petroleum based fluids are refined from crude oils. They are purified lubricant oils that have special additives.
Synthetic oils are new developments of the past few years.
Although they are compatible with normal transformer oils and petroleum fire resistant oils, they offer several advantages. They have much lower viscosities than petroleum oils, while offering the same degree of fire safety. They are clear fluids, which makes them easier to work with. In addition, they have much greater resistance to oxidation than do petroleum products. Transformer Cooling Transformers that were designed for use with conventional transformer oil will run warmer when filled with a fire resistant oil.