Your clutch pack consists of fiber disks and metal disks. They both wear over time and will deposit very fine fuzzy material on the drain magnet and also inside the cover. If that's all it is then I would consider replacing the clutch pack if it shows that it is needed, ( you can find the specs for what their thickness range should be before replacement ).
Your post doesn't say how many miles/kilometers on the engine or if you had any clutch issues before the tear down.
I like to use the KISS principle when dealing with motorcycles. Often it is the simplest thing that is causing the problem.
If you still have lots of room on the clutch adjustment and no slippage, I would just change the oil and you'd be good to go.
1971 BMW R75 project,
2013 Suzuki DL650A