My pleasure Martin.
In all my years of engineering that is the strangest looking sump oil I have ever seen, especially that black residue in your rocker covers. My own GS's have ran on everything from Unipart 20W50 mineral through bulk drum Chevron and Fuchs 10W40 semi-synth to Castrol Power 1 Racing 10W50 fully-synth and have never looked anything even remotely like that.
My suspicion is your Hi-Flo oil filter had run out of filtering capacity, was in bypass and the oil had then become exhausted and overloaded with combustion deposits. A good job it had an internal bypass, ISTR a few Mk2 Golfs chewing through cranks due to cheap pattern filters with no internal bypass valves and poor servicing schedules. Luckily there were no obvious wear deposits and now with fresh oil and filter things should clean up and move any remnants into the new filter for removal. I hope your camchain tensioners get a wash out with fresh oil over time.
The next oil and filter change should finish the job of flushing plus going to 20W50 should quieten things further. As for the previous service stamps, I call bullshit, the previous owner was had by somebody (presumably cheap oil change and no filter).
I'd also say you got lucky by doing the sensible thing and not continuing to ride it if it knocked when hot. That watery oil would not have lubricated very well the hotter it got and I'd wager damage would soon follow. Oil is always cheaper than engine work.
Thinking about it, i dont recall the residue when i did the valve clearances last time - although it may have been there and i didnt twig what it was.
I only use the centre stand to park it,
Only other thing i can think of, is there is a high spot on the inner cover which is allowing oil to gather there and not run back into the sump.
then when you park and the engine is cooling, the weak thing stuff i had in there is standing and baking onto the cover.