The hinge is rather flimsy, made from a material that corrodes easily, and they are assembled in the factory without any protective coating or lubrication.
By far the best way to lubricate them is to take the handles off, which means removing the door cards. If they break, you'll have to do this anyway. Then any decent quality grease with good water & corrosion resistance will suffice. This should also be done whenever a new hinge is fitted or it'll soon go the same way as the original.
If you're going to lubricate them
in situ, I'd suggest a silicone spray through a straw to help you get the lubricant where it's needed - something like
this. WD40 is a poor choice as it will both attract dust and attack any rubber-based seals in and around the handle.
Fiat could have prevented this from happening either by making the hinges out of better materials, or by lubricating the hinges in the factory.