Mark,
While I am normally not an advocate for an UOA, you fit the bill. Mobil EP is guaranteed for 15,000 miles or one year (whichever comes first). This doesn't mean that EP can't go over 15,000 miles or one year but when you extend an oil change interval beyond what is recommended and then you need to err on the side of caution and do an UOA. I think there is a good chance you can go two years but I have serious doubts that you could reach three.