You should not use either a petroleum based product, or WD-40 for lubricating the sliding ring tensioning mechanism. This is because both can soften the plastic, and ruin the stick. Personally, my own experience with the Xseries was that it didn't last long enough for that to matter, but I thought I'd mention it.
When I had one, I used a dab of Lithium grease. Others have suggested using Pam, the cooking spray.
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