We have an 03 E320 with about 70,000 miles now. I did a UOA a while back which you can search for. You will see that the oil holds up well. I don't know how much oil the crankcase in your engine holds, but it probably is something like 8 quarts.
If this is going to bother you, then do an oil change at 5000 to 7000 miles, and then stick with an annual oil change or 13k miles. If you are going to do the oil change at the dealership, be prepared to pay close to $200. If you do it yourself, it still will cost you about $80 to $90 just for the oil and the filter.
Do a UOA like benjamming recommended after the long drain interval or 13k miles to reassure yourself.
Don't worry too much about it. Life is short. Enjoy your car! The car is not eternal, especially with all the electronics in it making repairs after about 15 years of age or less cost prohibitive.