What causes black oil in a jet engine?

Answer 1

The same thing that makes ALL engine oil black. -Engines, especially jet engines have many finely engineered metal surfaces rotating against each other. -All that is between those speedily turning and tight fitting surfaces is engine oil ! -Friction is inevitable - so high oil pressure or oil with polymers is very necessary. In many parts of your engine, minute particles are burnt and carried away rapidly by the oil, - some is caught in the filter - some is invariably too small and so mixes with the oil and turns it black. -This is why we do frequent oil and filter changes.