What type of grade of oil does a Mercedes 2005 C 320 take

All it has in owners manual is: "Only use approved engine oils and oil filters required for vehicles with Maintenance System (U.S. vehicles) or FSS (Canada vehicles). For a listing of approved engine oils and oil filters, refer to the Factory Approved Service Products pamphlet in your vehicle literature portfolio, or contact an authorized Mercedes-Benz Center"

you did not say what the mileage is. As a general rule - use a good quality Synthetic Blend and you will be just fine. IF it is a high mileage vehicle and has not been on a steady diet of Full Synthetic, putting in Full Synthetic now could possibly lead to increased oil consumption.