Thought I would bump this thread. Big fan of Charnley Forests. They are easy to identify and there does seem to be a lot around. I have found them easy to use, effective finishers.
Regarding the question of oil or water - it seems to vary by stone. Fortunately it's pretty easy to tell once you clean them up. The stone on the left performed much better with water, the one in the middle with oil. Once I had stripped the oil out of both, the centre one was much more raspy, even after further lapping and burnishing.
I get a lot more meaningful feedback from the oil stone, which makes it useful for refreshing a blade because it immediately shows up more serious work needed. I use 80% Bisley gun oil 20% WD40.
The water stone works nicely with water only. It puts a very sharp soft finish on harder steels. Great in a progression following Welsh stones.
I have found that the finer Charnley Forests need a progression of lapping, even finishing them with 2000 grit silicon carbine wet and dry on a glass lapping plate.
The maroon inclusions are noticeably softer. When you lap them heavily it looks like the stones are bleeding. I got rid of one stone because there were too many large inclusions across the honing surface.
In comparison with other stones, I have found them less glassy than Llyn Idwal and much easier to read than a coticule.
That's my limited experience with them.