Yesterday I tested my 8 with the charger and charging cable of an 8.1 to see if the problem with my phone is the USB-C port or the cable, or the charger. I was very surprised to see that the phone did not charge at all, with either the 8.1 cable and my charger or the 8.1 cable and charger combined! I forgot to check using the 8.1 charger and the cable for the 8, but I imagine it would have worked.
The cable for the 8 is USB 3.x, it has a blue plastic piece in the USB-A plug. The cable for the 8.1 doesn't have blue plastic and therefore I assume it is USB 2.x. I have previously tested another USB 2.x to USB-C cable and that also didn't work, but I assumed it was because the cable was broken.... but now I'm not so sure.
Does anyone know for certain if the 8 will only charge with a USB 3.x cable? Why is this?
I also note that Qualcomm use a green plastic insert in their USB-A plugs to indicate QuickCharge compatibility, but this is missing from the 8's cable and charger...