You want a pex rated for hydronic use AND it should have an oxygen barrier. The specific version you choose is up to you. O2 can diffuse through the tubing wall if it doesn't have the barrier. An air eliminator only extracts bubbles, not dissolved O2. Dissolved O2 will react with the metals (especially iron) in the system (like the body and maybe impeller of the circulating pump). This isn't an issue with potable water, but can be critical in a closed system.