Need answers..94 mercedes c280 issues

Could be a bad ( mass airflow sensor ) my old c280 had a similar issue and I was able to clean the sensor with ( CRC mass airflow senor cleaner ) and it solved my problem. However it does require a special tool to remove, I just used a dremel tool and turn the bolts into flat head screws wich worked great. There are several good mercedes forums that have great info on this as well.

you need to check wich injection system does it have. if it is the cis system it has a plate to measure the air mass this needs to be serviced (cleaned) check that your maintenance is up to date ( spark plugs, cables, ignition parts, etc). please post more information if you need more help.

When was the last time the car was properly tuned and maintained. Black smoke indicates that it is running rich ( too much fuel, not enough air), the fact that you are experiencing high fuel consumption confirms this. Check all the systems that would contribute to an engine running rich. (worn spark plugs, dirty fuel injectors, old plug wires, defective ignition coils, faulty o2 sensor)

fuel in the oil often indicates an injector leaking. Will make the car run real rich too