I'm pretty sure you cannot check the oil level through nav on the X3 - simple because the MK IV doesn't have that function. You can on the 3 series nav, thats fine - because it has iDrive.

Either car though, like pseto said, you should be able to check it through the OBC... Or maybe through one of the hidden tests on the speedo?

E90post would probably have an answer for the 3 series - your the first to ask on this board so ?