Elastomeric O-rings in API 6A wellheads use HNBR, FKM, or FFKM for stem, bonnet, and body seals. AS568-sized HNBR (90 IRHD) seals 10,000 psi at 300°F, qualified to NORSOK M-710 RGD. Backup rings (PTFE/PEEK) prevent extrusion in >0.010-inch gaps. Trelleborg’s XploR™ H9T20 HNBR survives 720-hour 20% H2S aging with https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/