Once you find oil that "works," never, ever change it. The dirtier it gets the better it flows. As I've said often here, I've had excellent luck so far with straight 3-in1.
100%......next thing they'll be telling us is that we should adjust the valves.
In all seriousness, I've been keeping with a 3,000mi interval. The oil usually comes out almost the same color as it goes in. I also adjust the valves at the same time....even though there's not much adjusting.