do i have to use premium gas

If your engine has a compresion ratio of 10:1 or higher (which your car has) then you should use premium. If it has a turbo or a super charger then use premium. These are high performance engines... The tolerances are so tight that they are less forgiving when the combustion temp is off, or there are contaminants. If you use regular in a top shelf engine then you may encounter knock and a loss of efficiancy over time that offsets the cost savings of the gas and could lead to higher maintenance cost like plugs and seals needing more frequent repairs. Normal engines don't need it, but are not as efficiant. (less zoom for the boom) Even with normal engines I like to cycle the good gas every third tank just to run the detergents through, it keeps em clean and happy over time.

Premium is required in your vehicle. http://www.edmunds.com/mercedes-benz/clk-class/2004/features- specs.html