Since the oil drain is at the bottom of the differential, I don't believe being absolutely level is that critical.
Filling is done by the upper hole and full is determined by when the fluid starts to spill out or by measurement of the amount of fluid.
Tip: Don't drain until you have removed the upper bolt- If you can't fill it, don't drain it.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)