I know very little about the E36, but I know it is not really all too far off from the E46. If it says you need a specific O2 sensor for a '99 E36 M3, I would trust that.
I do not know exactly which location it is, but I know it is post-cat. If i were you, I'd check E36 forums. If nothing comes up, I'd unplug the wires from the post-cat sensors and see if "Probe 4" comes up for a secondary error (probably something along the lines of 'Lambda Probe (# here) Signal not plausible'). If you get a matching number, then you know just which one it is.
SEE SIG, DOGE IT ALL
My opinion is the opposite of your opinion, even if you change your opinion to my opinion.