Remember that once you change the oil of your car, you are removing all the crap that has contact w/ the oil. The oil itself will last for 20,000 miles but the crap that goes inside the oil makes the oil bad. Slight amounts of water could be inside causing rust. Ash of fuel burning.
I use synthetic in my new cars but I wouldnt waste the money for synthetic in my 300E. I just put a bottle of Mobil 1 in the 300E, for the extra startup protection.
If I were you, I wouldnt be putting Mobil 1 in my car. Try Valvoline MaxLife or run a blend of dino/synthetic like me,