Talk to these people for advice.

You only need to drain enough oil to get below where it is leaking out. If it is the tap changer's external operator leaking, it probably needs bushing packing or replacement. For a transformer this small (260kVA), it is generally unnecessary to vacuum fill unless you are going to leave the core & coils exposed out of oil for an extended period of time (days -weeks). At that voltage, the little moisture that could be absorbed into the winding insulation shouldn't affect its qualities measurably.

The work should be done in dry weather conditions, in a protected area so that water, dirt, dust, or other foreign matter won't get in the transformer. It is good to have some bottles of dry air available to purge the inside of the tank while the work is being done. It keeps out excess moist air, as well as making the inside breathable if someone needs to stick his head in there while working.