Oil/Filter: Time vs Mileage, and oil viscosity (two questions)

We will be putting only 6k miles per year on our 2005 Focus ST ZX4 . Is there any harm to just changing the oil and filter every 5k miles, vs the recommended 6-month schedule. At our mileage level, we would be changing oil about every 10-11 months. The car is operated in a relatively dust free region, and consists mostly of 3 to 20 mile trips, but almost no stop-and-go commuter traffic. I use 5w30 wt. Mobil One synthetic on my 1997 Crown Vic , which I realize is not an extended range oil, but I have had good luck with it on that car on the 5k interval, vs by the 6 month schedule. Also, I will check the owners manual for viscosity, as I don't yet know what the recommended weights of synthetics are for the Focus. Any of you use synthetics for the 2.3 liter dohc engine? If so, what weight oil? Thanks