Steel Metric Flareless Fitting, 20.00 mm S Series Profile Ring

420 bar is the rated maximum operating pressure for this fitting, so you're right at the ceiling. The S Series carries a 4:1 safety factor built into that rating, which gives you meaningful headroom against pressure spikes. For steady-state operation at or near that figure, the forged or drawn high-quality steel construction holds up well. Just make sure your tube OD is exactly 20mm, since the profile ring is sized specifically for that dimension.