Do you have an owner's manual? The API service classification and viscosity range for the ambient temperature range you will see are listed in the onwer's manual, but the listed API service classification has been superceeded. It is currently "SL" for spark ignition engines.
A 10W-30 should be suitable year round for the temperatures you will see in St. Louis. There are all manner of myths and hype about motor oils, but the bottom line is that the only two things that count are the API spec and viscosity range. Within a given range of viscosity and API classification, there is essentially no difference between brands, but manufactures spend tens of millions in advertising trying to convince us otherwise.
The primary benefit of synhetic oil is longer service life due to the base stock's higher resistance to oxidation, and modern cars that are OEM filled with synthetic have oil quality monitoring systems that may not signal a change for up to 15,000 miles. OEMs also specify a time limit - usually no more than one year regardless of mileage. The interval for your car is 7500 miles, and I think the time limit is six months. Again, check the owner's manual.
On a older car without an oil monitoring system you don't know how much longer you can go on synthetic, and without running longer service intervals there is no real benefit for the higher cost. This is particularly true for cars that don't see rapid accumulation in mileage and reach the time limit long before the nominal mileage limit.
Thus my recommendation is to continue with Mercedes recommended oil change intervals stated in the owner's manual paying attention to the time limit and use a conventional mineral based oil with a "SL" API service classification in a viscosity range that is suitable for the ambient temperatures your car will see until the estimated date that the oil will require another change.
Since you report you will be driving about 7500 miles per year, my recommendation would be to change the oil twice per year - fall and spring. Use 10W-30 in the fall for the cold winter months and 20W-50 in the spring for your hot summers.