I'd first make sure there's no outside of the folding mech. issues. Like a cable that's binding/hindering the folding movement. I would add lube to the hinge directly. Placing the bike in whatever position that makes it easiest to access the hinge. Don't forget to lube the latch or clip device too. It may take some cycling through the folding action to have the lube fully penetrate the hinge. Once a thin lube does get in then follow up with a thicker lube. Periodic lubing will keep the hinge from getting tight in the future. Just like one's car needs the oil changed even without much use over time so too a bike needs periodic maintenance. Andy.