I sometims need to add oil between oil changes. My garage uses BMW 5W30 synthetic. If I add oil, do I need to use BMW oil or can I use Mobile 1 or Castorl Syntec 5W30 synthetic oil.
BMWs are very specific on their oil requirements, I would buy several quarts of an oil that fully meets their specs. I imagine some synthetics do, some may not.
What year is your BMW?
Depends how close you are to your oil next oil change 
If under 2000 miles in a large sump of a BMW anything oil will work. If you are at the start of the oil interval you likely want to add the proper stuff that meets spec.
Mobile 1 and Castrol likely make a synthetic that meets spec.
If BMW thought that they could get away with it, they’d sell cans of BMW Spec tire air. Their oil should have a spec number. If you can get a synthetic that matches that spec number - and most likely you can - you’re in! If BMW is so picky about their specs, how come their cars breakdown so often? Could it be you really do need their spec air in the tires?