Lubricating the cables

I just bought one of those cable lubers and was wondering if any of you have used one before. I bought a '94 last summer with 30,000 kilometers on it and have no idea what maintenace has been done on the cables. The luber has 2 screws on top and the rubber insert is wide at one end and very narrow at the other (is that where you would spray the lube in from?). And what kind of lubricant is best to use for this??

Any help at all would be appreciated.