No need to drain oil. The steel pipe welded on the flange of the dehydrating breather is connected to the top inside the conservator. Therefore, the dehydrating breather absorbs the moisture in the air above the conservator. Not connected to transformer oil.
Don't worry, remove the dehydrating breather (of course. There may be a little oil in it. But still be careful not to let a little oil spill on your body.