[TDF9]

Recently brought a 2008 328xit with a 6spd manual, 135k miles. Been working my way through backlog of repairs and maintenance, and it's time to deal with code 54c8.

So I changed the transfer case oil, but code persists.

Weirdly:
* code immediately reappears after I clear it in INPA, even before I start the car (much less actually drive it).
* and the error detail includes the mileage when the error appeared - and it's always several hundred miles before / less than the current mileage.

Questions:
1) is recalibration needed after transfer case oil change FOR MANUAL TRANSMISSION cars? (Prior posts on this left me unclear)
2) can I do this via INPA or is ISTA the only way?
3) I recently replaced front driveshaft (on its last legs) - relevant in any way?
4) what to make of the immediate recurrence? I understand the error code results when the target voltage doesn't pass through the t-case oil... but is that even measured before the car starts?

Thx!