You never say how many miles are on your final drive. I assume a 1999 model may have quite a few miles. It will fail eventually without a rebuild. I was lucky and got 65,000 miles from mine before it failed. All of a sudden you hear a noise in the final drive you pull over and oil starts coming out. All of the drives in the LT were built with too much pre-load. This causes the metal to fatigue and then the crown wheel bearing begins to come apart. The pieces get ground up and puncture the seal and get trapped by the magnet.
If you are going to keep your LT, plan on rebuilding the final drive sometime in the future. As others have said change the final drive oil every time you change oil and pay attention to the magnet on the drain plug.
Alex
I disagree with this statement above:
"... It will fail eventually without a rebuild. I was lucky and got 65,000 miles from mine before it failed."
There are many cases documented here (of K1200LT models) and on the K1200RS forums and on the R1150 forums (other sites) of rear-drives with more than 100,000 miles without a rebuild. Mine has 95,000 miles - bought the bike new in 2002 and do ALL of my own maintenance.
These evidences do not mean that BMW did NOT have a quality / assembly control problems with these rear-drive - of course they did and they never admitted it. But please do not make a blank statement that all these drives will fail before such-or-such mileage. Quite a few have been properly done at the factory (properly shimmed).
I agree that if you buy a used K1200LT with unknown maintenance history, it is good prevention to have the rear-drive rebuild by competent tech such as Dave (Saddleman). However, to say that these will all fail is stretching the truth (or available stats / facts).
Another very good option: An owner needs to keep an eye on it with regular gear-oil change - a 20 minutes job that requires only 0.25 liters. Inspection of metal shaving on drain plug AND opacity of the old oil wlll give advance warning in most cases.