I change the dino oil at 6000 miles.
I usually change synthetics at 8,000 miles.
I did run Mobil 1 out to 10,000 miles twice, and I think Mobil 1, Redline, Amsoil, Edge, and several other synthetics can do 15,000 miles in some cars.
The car is a 1996 Toyota Avalon, and I have experimented with many oils in it.
I do not usually go past 8000 miles as there are other mechanical issues I need the mechanic to look at anyway, like brake linings. I have thought about using Edge out to 15,000 miles and CHANGING THE FILTER AT THE HALFWAY POINT. I think the amount of dust the filter can hold is the biggest issue. If it gets full, the best oil in the world will still be laden with dust, causing a lot of wear. But changing both the oil and filter at lesser intervals, using the cheaper dino oils, is about as economical as my mechanic is just two blocks away.