Hi Viney,
This sounds like a silly question but are you sure it is dark brown or is the oil level too high? This is surprisingly common on BMW forums where people report they cannot see the oil level but when they drain it they can then see the window is reasonably clear.
Assuming that is not the case and it really is brown it says to me you have a very dirty engine, have you owned the bike long and what oil are you using in it? How often have you changed it?
If the level is not too high and have a dirty engine then I would be giving serious consideration to draining the oil and taking the sump right off (make sure you have all the new genuine seals and stuff to do the job before you start. Clean the oil pick-up, Roys had some strange white stringy stuff stuck on his pick-up and clean out the sludge and carbon from the sump.
I would then consider replacing the window whilst the sump is off and then use a couple of changes of filter and good diesel engine oil like Shell Rimula R3X with good long hot runs to clean up the rest of the engine gently over a few thousand K each time.
Regards,
K-BIKE