There are some differences between various pads on the HD800.
The differences are not huge because the pads are kind of shallow and the metal mesh around the driver takes care of the needed 'leakage'.
For the HD800(S) this is not the task of the pads. In most headphones it is just not in the HD800(S).
This is where the HD820 is very different and has very different pads.
So you can use other pads but personally, when having bought an HD800(S) I see no reason to skimp on replacement pads (as well as the headrest) and simply buy the original.
The org. pads changed a bit over the years anyway.
I ordered very similar looking (but poorly fitting) imitation pads that keep 'popping off' by themselves because of poor fit. They sound very similar but not exactly the same.