I've been getting not so good fuel economy lately. Between 8 and 11l/100km. Not quite happy with that for obvious reasons. I was wondering if maybe my 02 sensor needs replacing but not sure and would rather not spend around 100 euro to find out.

О2 sensor can be read with a simple voltmeter.That way you can tell if it's bad or in good condition.
If you want i can tell you how to do it.
Cheers!

Unfortunately I don't have a voltmeter but I wouldn't mind you telling me how, for future reference.

Z5 engines are equiped with pre-heated 4 wires O2 sensors.2 wires are for the built in heater, 1 is for Ground and the 4-th is the signal wire to the ECU.
You need to know wich one is the signal wire and read it's voltage.
The catch is that you need a voltmeter that has a refresh rate about half a sec (500 miliseconds) so to be able to cope wit the O2 readings in drive mode.If not, a normal volt meter with 1 sec refresh rate will average the measured values.
l'm running one sec refresh rate vlotmeter in my car to be able to watch the values the o2 sends to the ECU.
Here is one video with it flipping from lean to rich on idlle, staying rich on acceleration and showing lean on deceleration when on gear(foot of the pedal):

https://www.youtube.com/watch?v=-4u9dkciIXI

If you have questions, ask.
Cheers!

Edit:
https://www.ebay.com/itm/Voltage-Displa ... qbTBsaLY9w