All automotive oils MUST conform to the API (American Petroleum Institute) "Engine Service Classifications" (SA, SB, etc., etc.). API dictates that oils of a specific classification MUST be miscible between brands without deleterious effects. So the answer is that synthetic Mobil 1 5W-30 and synthetic BMW 5W-30 are COMPLETELY miscible/interchangeable.

For what its worth, I personally use Mobil 1 15W-50.