X3 on the filler plug first. Torch both first, then use the breaker bar, you don't want to risk stripping the bolt or cracking anything.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition

As I've said before, I think the "not being able to refill" thing is more a symptom of not having the right tools.

With the cover off and in a vice, you can apply heat and much longer breakers than you can under the vehicle. Something that seems "stuck" under the vehicle may be butter smooth after some light propane heat and 3' of breakers. Mine certainly was.

Okyoureabeast, on most diffs the fill hole also acts as a check hole. .1 litres of gear oil shouldn't make a difference but when filling differentials I always just make sure I'm filling up to the fill hole or a just a bit below. Never had a problem and this is how I did my 300D diff, kinda like filling engine oil, I always use the manufacturer's specs as a baseline and then really use the dipstick to tell me when I've got enough.

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'84 300D
'97 Jeep XJ lots of mods
'01 Subaru Forester