Original Message

Engine oil is designed to run off hot bearings quickly. 
Gearbox oil is designed to stick to gear faces. 
You can see this effect by cutting the top off a can of engine oil and gear oil when they're empty and seeing how the different oil run off the can walls. The Engine oil can is very clean after just a few minutes. 

As to how they're used depends on the aircraft manufacturer. 
One oil for the whole aircraft is just a safety feature so people don't put gear oil in the engine.