I designed the O-ring groove clearance in the DSV for size -028 O-rings, but I have also used some metric: 2mm x 35mm. It is a bit thicker (2 mm = 0.078 inches, versus 0.070 inches for standard O-ring).
Even with the thicker O-ring I have not had any issues, but molded parts do have some variations due to tolerance.
The thicker metric O-ring (2mm x 35mm) will create a tighter seal if the tolerance on the parts runs on the looser condition. The trade-off is that it can be tighter and harder to rotate if the two cylinders are already running tight to each other.
Good silicone grease lubrication is always critical. There is always the concern of too much grease collecting debris, but I have not had that issue.
I do not see any issues using the softer 50 durometer O-rings, but so far I have had good luck with the 70 durometer.
It would be interesting to try the thicker metric O-ring (2mm x 35mm) in 50 durometer, but I am not sure where I could find that. I buy most of my O-rings from Mc Master- Carr.
Buceador con escafandra autónoma clásica.