How much oil is on the bottom of the diff? Is it just a mist that could have accumilated over a period of time??
As for the DIY job the problem you may have is that the pinion has a collapsable spacer betwen the bearings which should really be replaced once the nut is loosened. This spacer is there to set the pinion bearing pre-load.
I have heard that BMW dealers just take the nut off and replace the seal then tighten the nut back up but this is not the correct way of doing it!
The correct procedure is to strip the diff out, change the pinion seal and collapsable spacer and re set the pre load.
To check the oil level the car must be horizontal (on a ramp or all four corners the same hight in the air) then it should just drip out of the filler plug. Dont over fill it or it may pour out of the breather/seals!!
If you want the seal changing in the "proper" manner give me a ring on 01652 648429 (office hrs)
Hope this helps!!!
