It is an XM 2.1 TD Exclusive

I am rather new on this forum but have studied different issues around the Xm for some time.
I do have a question concerning refilling of the automatic gearbox, when changing til ATF fluid!
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The automatic gearbox is a piece of luxurious value

The lifetime and functionality is strongly depending on the amount and quality of the red ATF oil.
The oil level has to be controlled frequently, and changes has to carried out regularly,
The engine and the gearbox has to be warm when letting out the oil / take care wear gloves.
An oil change, along with inspection and cleaning of filter, will empty an amount of 2,7 litres out of the gearbox. The gearbox and the converter has 7,8 litres in all, so the change is a partly renewal.
This is according to XM 350-00/1 / but remember to check the level / is has to be keenly between the marks on the dipstick. The marks have one level indication area for cold condition, and another one for warm condition (80 degrees). The last one is the most precise. > > To low fill up / to high empty out
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The change and the frequency is described in different manuals, but there is one point where I lack an explanation:
On top of the gearbox housing there is an airscrew (ventilation) and a dipstick.
In the manuals there are guidelines to use the dipstick hole for filling?
> > >>>Why is it unwise to use the airscrew for filling?
Hope to get a skilled answer soon

Best regards
Mich@el