BunaN (Nitril), Silicone, and Viton will all work. BunaN has inadequate high temp specs and hardens fairly quick, then the leakage starts. Silicone has good temp specs but poor abrasion resistance, and gets eaten up by pipe vibration as fast as the cheap BunaN orings get hard. Never use silicone in a dynamic seal application. Viton is the material of choice, lasts far longer, and is also the most expensive.
44x3