When I was replacing my drive shaft splines I left the final drive upside down and some oil leaked out of the vent  at the top of the final drive. When I put the shaft back together cleaned it all up and have had no problems while in use for the last 3 months.
Today after leaving the bike on its center stand in the sun (22c) for a week I found a pool (approx 20ml) of FD oil under the bike (on an unhappy friend's drive   ). It looks to have come from the top vent judging by the trail from top vent to bottom where it ran off the drain plug and on the rear tyre before reaching the floor. It has not come from the main seal as that side of the FD is not contaminated.
Is this common, what causes it and how to fix?
What is best to remove FD oil from slate flag stones?

Hi there, are you sure it is FD oil? I´ve seen this happens and it has been shock oil which drips over the vent.
Hope this helps
Best regards
JAP

Oh no. I have just replaced shock that did just that but mainly while moving.

just been out to check - shock appears to be free of oil.

K100LT Notts wrote:What is best to remove FD oil from slate flag stones?

kitty litter or pulverized peat moss according to the collective wisdom over at Garage Journal.

After checking level - it is still close to bottom of threads so OK.
I just rolled it 30 yards backwards into garage and more oil came out of vent (Photo)
I levered plastic cap off and found an O ring (photo) at bottom of cap and a small bit of plastic film in cap (photo).
I expect will have to go to BMW to look at a parts diagram to see if anything missing.
Photos are found via this link

https://plus.google.com/photos/108070380341695510358/albums/6028815715270417793

I will also change oil when back in Nottingham where my torque wrench is.

By doing a google search I found threads on other BMW sites where this has happened to others, so perhaps not unusual, but none give a definitive cure.

K100LT Notts wrote:https://plus.google.com/photos/108070380341695510358/albums/6028815715270417793

"You do not have permission to view this album"

Anyway mine has also spewed a little bit out but only enough to make the outside of the hub look a little moist. I wonder if I overfilled it as the oil level changes drastically depending on which way the wheel is turned, does it really matter? Are you cramming in as much as you can, Mr Notts?

I examined the parts diagram and discussed it with my local BMW service centre. They say nothing is missing but if you invert the final drive oil can get trapped in the top section and may come out of the breather. Nothing to do just clean it up and check no more oil is still trapped under the breather cap. None there so I hope this is now solved. But I have learnt to keep the Drive up right when greasing splines!

You can lever the cap off to make cleanup easier.  It will expose the spigot which you clean with a rolled up rag.

If it was inverted or slightly overfilled, the drive will spew some of the gear lube.  Altitude will also cause this to happen.

I had mine do the same thing one day, random as all get out.   Never did figure out what it was, and it never did it again.  I would say having the final drive upside down didn't help.  In my research I learned that sometimes the gears will toss oil around and then end up with no space between the vent plug and the oil, causing a leak.  Only thin I could figure, cause like you all the correct parts were in place and in good working order. 

I spin the wheel after I get home and set it on the centerstand, just as a precaution.