Very Dark Oil

Re: 2009. I used Mobil 1 for 7500 miles. There was no consumption but it was nearly black. Surprisingly, new M1 is light brown in color, making it easier to see on the dip stick.
A privious car, Mercedes ML500 used synthetic oil and at the recommended interval of 10000 miles it was still the color of medium colored olive oil.
Anyone have some ideas on why the oil is so dark?
Toyota has of late extended oil changes to 10k with synthetic oil and I will do that with this recent change. But, the dark, nearly black color?
Reminds me of the old days with leaded fuel.