Hot water and baking soda work very well. Always rinse thoroughly with hot water, making sure everything is rinsed -- including letting hot water run through the spigot -- because any flavors left behind will show up in your oil. Let dry completely.
Remember to wash your stainless steel fusti, lid, plug, spigot and rubber O-rings thoroughly before use. They should also be cleaned between each filling.
You only need to apply the gas when you open the top of the fusti. As long as you are using the spigot, no oxygen is introduced, so there is no need to add inert gas.