0W-40 would be better if you can get hold of it, retaining more fluidity when cold and more viscosity when warm
Yes, it is OK to mix different oils.
It is generally not recommended but it is better to have two different 229.5 oils then too little oil...
But don't top it up if it's on the Min level, only if under (when hot).
It will probably be Mobil 1 then. 0w40 229.5
its a 1.8 engine not a 5.5 amg 0w40 is not a good move for that engine its like water i use it in the c36 and 5w40 in the c240 v6 and any 1.8 i ever had got 10w40