Shower door lubrication?

We have a sliding shower door, only 3 years old which does not slide nicely. It is a simple aluminium runner affair with no rollers. At times it screeches and at best it scroops. If we get hold of the door above half way up it also jams when opened. Gripping the handle below the half way point is the only way to ensure it does not jam. I tried lubricating the runners with silicon grease but this results in greasy black gunge running down the glass. SWMBO want me to change the doors but as they are tiled in with the tiles covering the screws that hold it together this will mean major work.
Any suggestions of a lubricant that will make them run more smoothly and not produce the black greasy stains?
You can see the offending doors here, http://share.ovi.com/media/Muddymike.Housechanges/Muddymike.10480?sort=5
Mike
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Muddymike wrote:

Mike,
Is there any adjusting mechanisms on the door to lift it off the bottom guide rail.
Cash
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Sadly no.
Mike
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