the oil seal has a spring that holds tension to the hub. over time, and this is lots of time mind you, this pressure will cause a seal to wear into the hub creating a little groove.
this groove could allow oil to seep past even with a new seal.
there are 'fixes' for this on more common vehicle makes, like small block chevy and ford motors, that consists of a thin metal sleeve that gets pushed over the snout to create a freash surface for the seal to seal against.
there are (apparently) similar 'fix-its' for the rear main sel as well, but i do not know if there are any made for the bmw motors. (i did not bother to check, but i'm sure a quick google will reveal any possible repair kits out there)
most repairs for the oil seal do not require the replacement of the hub. only if it is grooved.
a regular seal puller should be sufficent to remove the old seal.