Textured walls are tricky – too short a nap and you get patchy coverage, too long and it piles on the stipple. For standard orange peel/knockdown textures here, I find 10-12mm nap works best – it gets into the little dips without overloading. I switched to microfiber sleeves last year because they hold paint well, release evenly, and don't shed lint like some wool ones do. Got a pack of 12mm ones and they've been great for two full rooms – smooth finish, less rolling effort, and easy to clean. Cost around €20-€30 for a multi-pack, which lasts ages if you don't trash them. I grabbed mine from https://www.paintlab.ie/collections/12-roller-sleeves – they have good options in different naps with solid reviews for interior jobs. If your walls are on the smoother side, drop to 8-10mm, but 12mm has been my sweet spot for most houses. Test a small area first if you can. Makes a noticeable difference over cheap ones!