For mineral transformer oil in a vacuum dehydration/degassing unit, heater setpoints are usually kept in a moderate band: enough to drop viscosity and speed mass transfer, but not so high that you accelerate oxidation or «stray gassing.» Many service practices run around 60-70 °C for efficient vacuum drying/degassing, while CIGRE guidance also notes that oil treatment can be effective even at ~40-50 °C if the vacuum and process are set correctly. In the field, a common rule is don’t push much above ~65 °C unless you have a specific reason and tight control, because higher temperature increases aging rate.