l gave it for an engine tune up, and filled the tank with 91 Octane gas (recommended by the company) and it consumed 16 lit/100km.

Is that reasonable?

you are luck it even runs with that many kilometers on it :)

Great car. Never mind what the other guy said. l have an 05 legacy GT wagon & it turns in around 12L/100km. That is with a mixed city Hwy driving. If u have the tires inflated correctly u should be in the 11-12 or better. The Awd system adds friction & can cause higher fuel consumption than a 2wd car, but it should be better than that. Check the air filter & change the oil to synthetic. That should help. Other than that get a tune up with new plugs etc. That car motor should be good for another 100,000kms if u keep the oil clean. Enjoy the snow this winter. :)