It is possible for oil to pass between the axle and the ring that runs against the lip seal. It's normally caused by someone with no idea removing the bearing and in doing so, damaging the axle.
You may need to remove the axle and take it to a diff shop to have a new ring pressed on after they linish and polish the axle surface beneath it.
Are you sure you are not damaging the seal when you fit it? Are you using a large socket to press/drive it home?