If you are talking about residue on the sliding portion of the forks you have a leaky fork seal. Pull off the outer dust cover and wipe the seal. Ride a short time and check the seal, it will likely have a drip of fluid on it.
If the forks have never been serviced at 20k miles they should be dismantled, cleaned, and any worn parts replaced (bushings and seals at a minimum I would guess). Inspect the chrome sliders for any hint of nicks or imperfections that would have nicked the seal to start with. Sometimes these can be polished out very carefully with 2000 grit sandpaper, hand rubbed in 45 degree cross-hatch directions between horizontal and vertical.
This would also give you a chance to change springs and install any type of internal upgrade if you want.
These are not difficult things to work on if you have any special tools that are required for your particluar model. You could buy the special tools for what it would cost to have them done at a shop.
BTW, the used fork oil will likely be black as coal and will stain anything it comes in contact with, so be forewarned.
1985 Honda VF1000R, 1990 Ducati 851, 2008 YZ450F