I use lacquer thiner but be careful and use it sparingly.Break cleaner with Tetra.....blah blah (some chemical name I don't remember). With this chemical component, you even use it on interior upholstery without any damage to fabric or the glue under the fabric. Test an area first.
I have new fuel stain very similar to yours. I had temporarily put on an old fuel cap that I figured out later didn’t have the small O ring on the shaft. Not a fuel cap I had ever used and wasn’t going to fly with. The tank was full enough that the cap was dipping into the fuel a bit so it wicked up the fuel and bled out. You might check your small O ring and check the adjustment of the cap clamping force on the large O ring.So MEK did the trick without excessive amount of elbow grease. It did dull the finish a tad, but polishing will bring it back. I will investigate fuel vent. FIrst step will be to locate it. It wasn't obvious on my first look-over.
You might check your small O ring and check the adjustment of the cap clamping force on the large O ring.