Well, first of all it's spelled and pronounced Cummins, and it is an engine manufactured by that company.
Its actually called a cummins diesel because that's the manufacturer of the engine. They are widely used because of the name and reputation that comes with them
Cumming made/make many different engines ranging in power from a few to hundreds of horsepower.
If you mean the KTA-600 Cummins (from 1970's)it was 600hp@2100rpm and torque was 1650@1600 rpm. Displacement was 1150 cu.in.
It is either bad door seals or the turbo that you are hearing.
Your 5.9 Lts holds 11.78 liters of oil