1. If you're going to be picky about your oil, you should probably consider going with one of the diesel oils - Delo, Rotella, Delvac, etc. If you're used to the cost of Mobil 1, changing to the synthetic Delvac (Delvac 1 5W40) wouldn't be a bad idea. It's supposed to be really good stuff, and I'm using it in my 300E right now (trying to clean out my crankcase, will probably go back to Mobil 1 15W50 later). You could certainly use Mobil 1, but the other ones are better for diesel applications.
2. Dont' know about this one.
3. Depends on your driving style/habits. I spend a lot of time on the highway (or at highway speeds), run synthetic, and still change my oil every 6000-7500 miles.