All of the whetstones sold at the shop are Waterstones, do not put oil on any of them, you will ruin your stone. Holding the knife at a consistent angle while sharpening is one of, if not, the most important thing to consider when sharpening; thus, maintaining a flat stone is of equal importance. Pick up a truing stone to maintain a flat stone in order to achieve the best results.
If your stones are super worn out and completely concave using a truing stone will take hours to flatten them. Consider purchasing a King flattening stone to get the job done quicker.