Why is viscosity of transformer oil in cSt important?

Viscosity controls how easily oil circulates through winding ducts and radiators. Lower viscosity generally improves convective heat transfer and reduces pump power in forced oil systems, which helps keep winding hot spot temperature down. If viscosity is too high at low ambient temperatures, circulation slows, hot spots rise, and paper insulation ages faster. Viscosity also influences gas bubble movement and particle suspension. That is why specifications include viscosity limits at defined temperatures and why oil selection may differ between cold regions and hot climates or between ONAN and OFAF cooling designs.