Suehiro Gokumyo 20k updated opinion.
My Gok 20k arrived just on 2 years ago, and at the time it was the 'panacea' for all honing issues. It was. And it still is the number one choice for people honing straight razors on synthetic hones as the final stone.
I have made however a few observations. There are 3 'Gokumyo' series stones from Suehiro, I have owned, a 1 micron 10k, a .75 micron 15k and the .5 micron of the 20000 grit stone. I have since parted company with the 10k and the 15k, as either or both could admirably fill the role of 'finisher' in their own right. I recall the comment that there is swarf on the 20k after only 10 or so strokes. This is where the true position of the 20k is mis-read.
Thinking that metal particles after 12 strokes on a 20k .5 micron stone is ridiculous. That dark streaking that can appear on the surface of the 20k, is not swarf, it is a part of carbon steel, 'Graphite'. It is very much like the lead in a pencil. As a test, take your Gok 20 and run the spine of a razor down the hone. Now you understand. Equally do a dozen stokes on a taped razor. Now do it un-taped, the dark on the stone is multiplied. As a result of this, I use the cleaning stone maybe 3 times just honing one razor, finishing. A couple of strokes with the cleaner stone, wipe it, and you have a slightly dis-rupted surface, and the end result is superb. So, to say that after a dozen strokes on your 20k, that there is suspended steel in the liquid simply is not the case. Still a great stone, just a tip, remove that black stria blocking the pores of the hone with the cleaning stone, you will get much better results on the 20k when it's cleaned of the graphite.