What is the name of this tool?

wd40 will break down the lubricant in the bb and you'll be left with a lubricant that is to thin

Okay

how do you know it is 80°C?

as soon as you put heat into something that has been treated you start to change its properties (..)

I normally use a small IR thermometer in conjunction with the heat gun.

I understand your point but actually I know heat treatment only from chainrings and bmx cranks, so mostly cro-mo stuff and not alloy like those Miche cranks I was removing, so I guess I should be fine.