I have some 16mm/10mm soft tubing with appropriate Bykski fittings. I need to order some more black compression fittings but the only ones I can find that aren’t shipped from China are Barrow fittings which are 3/8 - 5/8 which I understand are comparable. Can these be used?
Yes, it used to be that all PC water cooling parts were advertised in inches but then EK started using mm, some other EU manufacturers followed suit, then European retailers started doing it too (some even removing the inches and just advertising mm).
As a general guide:
5/16 ID - 3/8 OD = 10/8mm
3/8 ID - 1/2 OD = 13/10mm
3/8 ID - 5/8 OD = 16/10mm
7/16 ID - 5/8 OD = 16/11mm
1/2 ID - 5/8 OD = 16/13mm
1/2 ID - 3/4 OD = 19/13mm
You don't have to worry about mixing inches and mm as most manufacturers list their hose sizes as an approximation anyway, I.E EK's 10mm is actually 9.5mm and their 11mm is actually 11.1mm, it's very rare that tubing/fittings in PC water cooling is precise to the whole number. As long as nothing is out by more than a whole mm you should never have an issue (unless you're using barb fittings without clamps, not really a warning needed in 2020 but figured I should mention it for completeness).