Engine oil or transmission fluid hitting the cooling fan will be really evident. Even a small amount will spread all over the blades and transfer to anything that touches them. Just look at the fan from the engine side and it will have a wet film.
The first thing I would check for is a fuel leak along the injector valley, most likely a leaking or unclipped return tube somewhere.
First, Be VERY careful, and use a paper-sized cardboard feeler to make sure there are
NO high pressure leaks. along the rail, injector feed tubes, and fittings. Even a very tiny one can penetrate your skin, causing immediate blood poisoning, with potentially fatal results.
THEN you can feel around the fuel return lines to the back and all the way to the return fuel manifold on the rear driver's side of the intake "Camifold" whilst it is running.
If you have leaks there, GM Duramax use the same return tubes fittings, and injector-fitting clips at a fraction of the price of JEEP OEM.
I know it seems counter-intuitive, but others have encountered oil (fuel) splatter from this source, because air movement actually blows
foreward along the injector valley, and leaking fuel can both travel up the back side of the rear timing cover and be splattered along the top of the engine, and/or leak down behind the rear timing cover if it is not sealed to the head, and come forward along the fan pulley hub and again be flung around by the fan, but if this is the case, here again, the fan will have a wet film.
If your fan has no wet oily film, when the engine is off, feel along the rear top of the rear timing housing if there is wet oily film there.
EDIT: Wow I need to apologize for giving misleading info here. I completely overlooked that you have a v6 petrol engine, so the above does not apply. 
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