A 10w-40 synthetic oil is the standard but some people prefer 15w-50 for higher mileage Mercedes. Synthetic oil will protect your engine better than conventional oil, but it can be more expensive.
If your car has been using a natural oil originally, switching to a synthetic oil may cause it to consume more oil than normal. Using a synthetic blend first then transitioning to synthetic oil will help remedy this. There seems to be varying opinions on what weight of oil to use on a Mercedes Benz 300. Some prefer a thicker 15w-50 but it is always best to consult your owner's manual for the proper oil viscosity.