What I like to use in situations like this is liquid graphite instead of grease or something like wd-40. Like Berry mentioned, WD-40 is not really a lubricant per sae and the thing with grease is that it attracts dirt and can get gummy as it gets contaiminated. Liquid graphite is near perfect for something like this because the small graphite particles get inside the component in the liquid suspension and then when the liquid dries, it leaves just the dry graphite all over the place as a lubricant. It takes quite a bit longer to contaiminate since there's nothing really there for dirt to stick to.