A wet capacitor is provided, and the use of an insulation fluid composition in such a capacitor. The capacitor includes a package of a metal foil and a polymeric insulating film, or of a metallized polymeric film, wherein the insulation composition includes a synthetic or natural aromatic oil and a polymer. The insulation composition is configured to undergo a thermo-reversible oil-to-gel transition at a predefined gel-point temperature. Further, methods of producing such wet capacitors are provided, optionally including additional filling materials, and methods of sealing leaks in such capacitors.