[quote="drunkenmunky"]...the guy has put some oil in it and then it became smooth...[/quote]
They really need to be stripped down. It's a fairly common problem for the lower sliders to become stiff. There's a rubber sleeve that goes all the way through the slider hole. Moisture gets under the sleeve and surface rust occurs there. This effectively, reduces the hole diameter and stiffens the slider's action. The oil will only be a tempory fix I'm afraid, they need to be cleaned out and greased. I would get it done sooner rather than later before it turns from a 30 minute job (for both sides) into a 2 hour job like mine was last year
Mark
They used emery paper on mine. I couldn't bear to watch them beating my calipers in a vice with a big hammer trying to get the sliders out, so I went and played with the guy's pit bull