If you are mechanically savvy enough to get the damper off the bike you can do the refill yourself. See the excellent video of how to do it on the BMW Motorrad Club of Northern Illinois site.
http://www.illinoisbmwriders.com/service/instructional-videos/15-k-bikes .
In most cases I suspect that you do not need to replace the seals unless they are leaking severely. It is so easy that I think you should plan to add this to your annual maintenance schedule (recommendation from Kirk on the video).
Condition Note: before I started, it was obvious that my damper was not doing any actual dampening based on the back and forth handle bar test. It did not seem to affect my riding but I am also blessed with a bike that does not chew up the front tires. When I got the unit off the bike and manually moved the piston it confirmed that there was no dampening action and when disassembled the damper was extremely low on oil compared with what I refilled. My bike is a 2002 with 45K miles.
Hard parts when I did the procedure:
1) First problem encountered when taking off the front tire/wheel assembly so I would have room to fight with the bottom bolt that holds the damper on the bike. My bike has the BMW embossed front brake calipers. Unlike the Brembo embossed calipers, the BMW ones don't have room to just slide off of the rotors. I had to take the extra step of loosening the bolts that hold the front rotors then I had room to remove the calipers which allowed me to get the front wheel and tire off the bike. Yes there is also a video for how to remove the front wheel and tire on the Illinois site if you haven't done this before.
2) That torx bolt on the bottom of the damper is pretty much the worst piece of crap I've ever dealt with. Give it a try but plan to have to drill the head off. Really not that hard since the bolt seems to be made of the cheapest metal available. Once the head is drilled off just wait until you get the whole damper off then grab the remaining threaded shaft with vice grips and twist it right out.
Before you start this process, go to the hardware store and get a good grade (8.8 or better) hex head stainless steel bolt size 8 with a 25mm long threaded shank (from the bottom of the head to the end). I think I paid all of a $1.25 at Ace Hardware.
3) The oil that Kirk uses in the video to refill the damper was unfamiliar to me so I had to do a frame grab to see that he was using a bottle of Honda branded 5W fork oil. Bought a quart bottle off of Amazon for about $10.00 and after the first use I think I can do this process about 5 more times if I'm not totally wasteful. A lot of shops carry fork oil in that weight but wanted more money and they were selling a synthetic brand. I'm cheap, what can I say.
4) When I had the damper disassembled just like in the video I noticed that the end of the piston rod that sticks out forward normally had some built up grunge that I removed with a soak in WD40 and a soft rag. Then I looked inside the forward seal and noticed some crud there as well. I used a dull nylon stick to gently wipe the seal surface until the crud was removed without damaging the seal. I think this build up is probably common and creates the path for the oil to escape, just like in forks.
5) I found that I needed to fill and flush the damper several times to clean out the crud and sludge. Kirk mentions this in the video but I was surprised anyway. I think if I do this annually it won't be nearly as bad.
After: I could immediately tell a difference in the action of the damper on the bench and I could also see the difference once mounted on the bike. I think I can feel a little more stability on the road but not as much as I anticipated. Still, well worth the effort and now that I'm past that torx bolt it should be very simple next time around.
