2003 W203 C320 4Matic

Should I buy mobile 1 0w40 to change the engine oil?
Or what brand of engine oil should I get?
Somebody says I should have a mercedes 4matic oil..?

 

Mobil 1 is what MB uses... if you're unsure call up the parts department but Mobil 1 0w40 is what most use. As long as its MB spec you'll be ok.

 

Yes.

 

how many liters should I get for my car?
7liters?

 

Depends on engine but I use 8.5 or so usually. Do you not have your owners manual? It gives all the info.

 

Mobil 1 is perfect, get at least 9 quarts. I just buy it by the box. Get a Mann fleece filter if you can. I get them at the NAPA auto parts store nearby. It is pretty straight forward with the exception being the filter is accessed at the top of the engine.