I use engine/motor oil, about one drop or less, every six months or so. WD-40 will evaporate in a few hours, and grease attracts dirt and it will get stiff over time.

WD 40 and similar products will wash out very quickly, and do not provide a long lasting protection. Grease is too thick unless your hinges are equipped with zerk fittings.

A fine oil that lasts in the weather, keeps the hinge pins free and from rusting, is automotive transmission fluid. I drain out the residual oil in the bottles from my last fluid change, and apply it using the old syringes from the last round of shots the horses received.

It will flow to every part of the hinge, and provide a very well lubricated joint. It also works well on latches and almost anything that moves. The synthetic variants are longer lasting and even more weather proof.

Gard

I recommend about .2 ml of Synthetic Enduralube III.   It is a little pricy but it stays put no matter what the weather.  I even sprayed it with a 220 psi power washer and a propane torch and it stayed right there doing its job.  Its a little hard to find but check with Tractor Supply.  They usually carry it in single use packages. If you can get by the Blue Moon dealer they usually have it in six packs of 55 gallon drums.

Apply it with a Q-Tip, being careful to put it only on the parts that are joined.  It it runs off it will eat away the aluminum or stainless steel before you can say "spend another trillion of my children's money."  It will also cause your horse to develop a rash if the vapors accumulate inside the trailer so keep the vents fully opened.