I'd never run a synthetic for 15k, that's asking for trouble. Sure it can last that long, but oil gets dirty and going 15k is dangerous. 5-8K tops is what I recommend.
I too have free oil changes with my car for a year from Toyota, and there oil is pretty good stuff (any OEM manufacturer oil is).
Regarding Mobil 1, they do have higher quality oils then say Pennzoil. Ever watch those Castrol GTX commercials on how some engines fail early under severe driving conditions using other brands of oil. Well think about your investment in Mobil 1 that way, it will protect better then conventional oils.
In terms of pure synthetic, Pennzoil does have a great one in the Pennzoil /w Penzane (that's the only good oil they got). Mobil 1, Royal Purple and Redline is the top ones. Any other brands you can buy from the stores, usually suck.