I think that's oil. Fuel is a vapory white trail and smoke, well, you'll know it when you see it as it's much more pronounced.

The best thing about such an oil leak? Pause the game and move the external camera behind the leaking engine looking forward, you'll see that the oil comes out in droplets that swirl and rotate in the airflow ;)

Now thats the kind of awnser i like :)
Thanks