A. Some of them, yes. The Elements series of brushes (Rocks, all Clouds, Water, Fire and Ether) are designed for use with water-based mediums. They are possibly suited for oils depending on your oil painting style (since the spirits used in oil painting are damaging to the delicate brush hairs); they are not however suitable for acrylic, as the plastics in these components may damage the brush hairs. The brushes are suitable for careful use of gouache (which is fairly thick) but to bring out the best in them we recommend you stick to watercolour paints, either commercial or handmade. In general, we recommend using them for watercolour, at least at first. When they lose their fine points - as all brushes will inevitably do - you can switch them over to oils. Please wash them carefully after each use and they’ll last you a long time. The Old Goat series (all Old Goat sizes) was designed for inks and can also be used for watercolour - but not oils. The Humours series of brushes (Sanguine, Phlegmatic, all Choleric and all Melancholic) were designed specifically for oils, as was the Young Ox series of brushes (all Young Ox sizes) - although, conversely, they’re also suitable for watercolours. Don’t switch between mediums - once you’re using one medium, stick to it. The exception is when your brushes are a bit worn out and you’ve transitioned brushes from water-based mediums to oils. Then stick to oils, don’t switch them back to watercolours.