Never use WD40 on ANY plastic bushing! It dissolves the plastic and creates a worse problem.

Use the grease or a heavy silicone spray. Light silicone (one which simply puts a "sheen" on the surface) lasts about as long as a fart in a windstorm.

Some of my friends swear by INOX for lubing plastic/nylon/teflon slides but I haven't used it enough to recommend it.