@RhysU, I have a 99 R1100S with 27k miles, and your post made me check my bike. I did a 300-mile ride on Monday and I see similar drip+dirt on my rear wheel.
Checking the bottom edges of the rear drive boot, I have oil that collects then drips down. I'll pull back the boot later to see how bad it is, but it doesn't look like much.
Something else to consider is the fuel overflow hose. When I travel I like to fill the tank to the very top when I stop for fuel. My R1100S is black and I was riding in 95° heat with cloudless skies Monday. In my case, fuel may have expanded in the heat, overflowed, and created the crud on my wheel. Sorry I wiped it down before the photo, but you can still see some of the residue on the link rod.
