What is the best oil level for a 27 years old 617.912 Diesel engine?

A friend of mine told me that it's best to keep it between middle level and 3/4, too much being bad on the oil pump/system. From my experience the engine sounds better when the oil level's between middle and full level but I want to know what would be the best/recommended level for this type of engine. Note that it doesn't use up any oil so I need to get it right once/oil change.