Get an Oil Analysis done. It's really the only true way to know how long you can go and what oil and viscosity works best for your vehicle, driving style and enviornment. I can go 13,000 miles with 0W30 Mobil 1 extended in my Nissan Titan before the oil wears out (TBN is low) but only 8000 miles with the same mobil 1 but 5W30 so it pays to have the analysis done.
Oil Analysis