A little while ago i owned a mercedes c200 cdi, i had 500 miles left till me next service, however the car was continously asking for engine oil when i had around 700 miles left till the next service. So i thought instead of wasting a service i might aswell put the engine oil in myself. I called up a known uk supplier called euro car parts. They advised me that my engine can hold around 5 litres, i ended up buying 2 x 2 litre engine oil bottle from them and filled up the car. 200 miles on the way back home on the motorway the car is dead smoking. Smoke coming through every hole possible.
Just remember always one litre and if its still less your better off going for a service rather then spending more then 500 quid to fix it later