I just opened the game to look into this and the synthesis screen doesn't show your current life support level.General question as I never did this:
what happens if you synth life support?
If you synth ammo you see in the synth menu how much ammo is left. Do you see this with life support, too?
If yes, it would be a viable indicator...
In fact, I couldn't even get the synthesis screen to show how much ammo was in my weapon... It could be that it only shows ammo levels if you have more than one weapon that could take a refill, so you don't pick the wrong one. The ship I happen to be in at the moment only has one frag cannon and two lasers, so it's honestly the first time I've ever looked at the synth menu without multiple reloadable weapons. Thus I can't tell if that was a change that came with the new UI layout, or if it's never shown ammo levels with only one kinetic weapon.
But I'm getting off track. The point is that the refill screen doesn't show how much oxygen is left in life support.