Current consumption temporarily over 30 litres

Hello, dear people,

I've been the proud owner of an Audi V8 3.6 for a week and I'm new here in the forum.

Now to my concern... since yesterday, I had to find out that my Audi outside of town, when driving straight out and normal gas position, temporarily increases to more than 30 liters at the moment consumption and then also does not run as round as before, where the current consumption indicator has displayed something about 10 liters.
This morning on the way to work the same problem but only when the engine was warm.
Even mountain downhill with minimal gas position, it shows around 20 liters in these phases.

Of course, the whole thing is also reflected in the tank.
After 150 cozy extra-urban miles, the tank needle does not even show half.

Everything just a matter of attitude or a serious problem??
Thanks in advance for your answers :-)