The Dealer costs on oil changes are high because of the labor charge. Don't feel bad if you think you paid too much. If you want to save money, do the oil change yourself. It's not that hard, and you can get tips and pointers on this site to help you. Earlier testing with the 3.2 engine in the Mercedes cars showed the lower grade oils don't work so good. Use Mobil 1 0W-40. Remember that an oil change should last 6,000 to 7,000 miles not the regular 3,000.
Don't use a lower octane gas either. You will not only get pinging, and studdering, outside of other problems, but lower gas milage as well, meaning you'll use more gas, so who wants to buy more gas?
This is an unusal high performance car, it may cost more, but you can cut the costs down if you are willing to learn.