There's always a good bit of conversation on the slate post that I did way back, and that's good, I'm glad people are digging through the archives. Lots of good stuff in the archives.
I thought it might be a good idea to redo my test, albeit in a very limited way, on the staining properties of slate, at least in regard to oil.
See for yourself what the result is. No staining! If you look closely at the first sample with the oil on it, you may see, on the unoiled part, a few white marks which are scratches. After I wiped off the oil on the sample with dishwashing soap and water on a sponge and brushed the slate using the opposite side of the sponge fairly roughly, "wah laaa" the white spots disappear! If they do not completely disappear, then it is my understanding (although I have not done it myself) that one can lightly sand any significant white scratches and they will disappear.
Yes, slate is strong, VERY strong (and very dense)! Take a look....the yellow text, which is too small, says "dark line" meaning to please try to find the dark line on the second image too so you can get it oriented. Sorry, when I tried to rotate one image or the other, it looked much more skewed and a bit more difficult to see, so you'll have to reorient yourself to find the similar graining of each sample so you can see where the oil was on the second sample. Still, do your own test, on any countertop surface.