For C-class engines, the use of multigrade oils is recommended. They are made in liquid seasonal oil (e.g. 15W) and therefore it is not necessary to change them depending on the time of year (summer Winter). With the help of so-called thickeners, the oil is stabilized while hot.
"W" in the SAE designation means that the oil is suitable for use in winter.
Easily flowable oils are called all-weather oils, which, along with other additives, include components to reduce the coefficient of friction in the engine. The basis of the free-flowing oil is the so-called synthetic oil. When buying light oils, you should pay attention to the fact that it is approved for use by Mercedes (designation on the container).
Additional lubricants of any kind must not be added to fuels or lubricating oils.