Per previous suggestions, I've been using a soft cervical collar to prevent chin tucking and have started using REMzzzs mask liners to decrease leaking. Things have improved, and without this board I wouldn't be heading in this direction. Thank you Debra and SarcasticDave!
I was initially between 12 and 18 AHI. I think with a combination of moving to the AirTouch F20, the liners and the soft collar, with the kind help of people on this board, I've managed to get the leaking down to mostly minimal levels (with the occasional night that's worse than others - as you can see from the data).
The problem is I'm still get hypopneas, and my AHI is generally floating between 5 and 8. I feel a bit better, I'm dreaming again which is a good sign, but my persistant fatigue, gut and the OSCAR data is telling me there's more improvements to be had here.
I'm open to any further suggestions anyone might have. I'm not all that convinced that the outstanding problems are positional apnea in nature, nor due to leaks. As, the hypopneas seem to be present with or without the small leaks that are continuing to occur per the data. I'm convinced I've fixed the positional apnea with the cervical collar; it's completely changed the way I sleep (partner confirms this). But please do say if you think I'm wrong

Look forward to hearing any thoughts you might have
