Regardless of full synthetic or not, I change the oil on all cars about every 3K miles.
If I had at least a 20-minute commute (I don't), I'd extend it to 5K miles with dino, and 7.5K miles with synthetic, with a filter change and 1/2 qt. top-off at 3500 miles.
It's the real short drives that is killing my oil, because it rarely gets hot enough to boil-away the contaminants, it doesn't matter if I use a good diesel rated oil like Delvac 1300, Delvac 1, or Delo 400.
1988 360TE AMG