I will say this. I just changed the
oil in my 07 F150 for the 3rd time. There was almost 6K on it from the last
oil change and the
oil was good. I am have been using the motorcraft
synthetic blend that Ford recommended and the oil was not even completely dark. It still had a brownish color to it. I will be extending the time I take to change the oil. The Mustang however (same oil, same 4.6 mod motor), after 5K is dark, after 6K and I change the oil, I notice a difference in how the engine runs. Idle is smoother and acceleration is smoother as well. I believe there is no set time, and 2 like vehicles will perform differently.