It is essential to use (PTFE) pipe thread tape on polymer male thread connectors (ME15, M20 etc.). Use a small amount and wrap correctly on the male thread. Avoid using liquid thread sealer compounds on Buteline polymer threaded fittings. Do NOT overtighten as threads are tapered and therefore will tighten with less turns compared to male parallel threads. Hand tighten + a ½ turn.
No (PTFE) pipe thread tape or sealant is required with Buteline polymer female swivel connectors (F15, FE20 etc.). Just ensure that the shoulder of the female is directly presented squarely to a flat machined face before tightening. This avoids crossed threads and ensures a seal.