My oil, drained at 6-7K miles is dark at the end of that period. It takes probably about 1K to turn dark.
About 15 years ago I was using Mobil 1 in my EXP (remember those?) with the same drain intervals. I had to change the valve cover gasket at 90K. The entire valve train was very clean as well as the underside of the valve cover. Since that oil was a couple of generations behind the current Mobil 1, I have more confidence now with extended drains than before.
Again, I know many people who have gotten over 200K on the cars using only conventional oil. My father had a old Ford Grenada with 200K and never used synthetics. The car ran well before it got wrecked!
Lastly, conventional oil has been improved vastly in the past ten years as refining processes have improved.
