WD-40 is an OK penetrating oil, not a wonderful one so as was said it may take some time if the problem is that the pivot points have rusted. I use WD-40 often because it harms few plastics and doesn't have the objectionable lingering stink that some of the better penetrating oils seem to have.
If the WD-40 works it should be followed with a real oil... edit: lubricant... (I'm like Pigpen in Peanuts so dust and dirt don't matter) of some type. WD-40 is very light (it has been described as diesel fuel based) so it has no staying power.
I like to keep Hercules Lube Oil on hand. It is commonly used for lubing Bell and Gossett circulating pumps/motors.
It is readily available at HVAC supply stores. Has a handy long extendable spout. Doesn't spill easily. It works for many general lubrication applications. It has more body than 3 in 1 oil. That said, any motor oil in a squirt can will work for your step pivot point lubrication.
If the steps don't loosen up in a short time you should inspect them to see if the mountings have become loose or it is otherwise out of alignment which could cause binding.