10L/50km? That's (mathematical genius here) 20L/100km! Something is definitely not right, your ED is running waaaaaay rich...
I'm such a great help, I can say "yes, you have a problem there" but I can't offer an answer
Spark plugs, leads, head gasket, oxy sensor, air filter, timing, general tune, injectors, umm, any of these things could be the culprit (or culprits).
Maybe finding a deserted road & giving it a bootfull to blow out the cobwebs might help. I was averaging 400-450km per tank (although mine's a 5.0) until I gave the car absolute shit one day, and after that I miraculously gained over 100km per tank after that! I now do over 500km per tank around town/country driving, and 600km per tank freeway driving.