I think the confusion may be due to '12,000 grit or higher stone'. Grit ratings are for synthetic stones and whilst I don't own any high grit synthetics most don't seem to use slurry on them.

I do have one of the Welsh slates which was sold as being around 12,000 grit. I use slurry & water on this and dilute down to water for the finish. I have used it with oil, not wd40, I don't use oil & slurry.