oil consumption at 406

hi@all hab n 406break (bj97, 110ps). he needs at 1000km zt. 1l oil. the consumption was increasing in the course of the time. I reported this to the vehicle dealer early, but always the saying “look into the instruction booklet…”. who knows/has the same problem or knows what you can do. I already tried oil additives, unfortunately not indication. motor takes in the lower speed range also time-wise bad the gas (delayed, ruckelig…) car has 109tkm.