I would have thought FD cylinders would have been the cleanest? I believe all my locals are required to get quarterly tests. Not to mention, their woven SCBA tanks I don't think are allowed to be immersed in water because it gets absorbed. I would think they would want the driest air.
Negative ghostrider. Those cylinders are chained to the wall and not opened for years. It only take one filter failure from 3 years ago etc to pump in huge quantities of oily moisture. Grade D (surface use) air has much lower moisture tolerances than grade E.