My handbook says GL5 for gearbox and FD on a K100RS 16v, but the oil specs. suggest that GL4 may be more appropriate for the gearbox and GL5 in the FD. Any advice, please? And what's best for fork oil on this model? Mine appears to have almost no rebound damping at rest but rides OK.

Just use a good quality, multi viscosity GL5 Hypoid oil for the helical cut of the gears in the gearbox and final drive, whether synthetic or mineral based is down to your preferences, and you'll not have to think about it again.

The standard fork oils are either 7.5 or 10W. At the BMW dealerships I've worked in we've used brand-name 5W or 10W and the factory's Telelever 10W in most bikes. 15W is OK if you really like a hard jolt over bumps or you're fond of going back for seconds and even thirds at the all-you-can-eat buffet. But it has a tendency to make a weak seal leak.

I use 5W in my 280cc per leg capacity of the K75 'S' forks retrofitted to my K100RS. I like the supple reaction over bumps and the fork springs are already quite jostly so the lighter weight absorbs a few more of the little ripples and still does its job. Your bike will take a larger amount of fluid which is published elsewhere.