Body bonnet gaskets in API 6A wellheads are typically ring type BX or RX stainless with PTFE or elastomeric coating. For extreme service, metal end-cap or spiral-wound Inconel with FFKM core provide absolute sealing at 20,000 psi and full temperature range https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/