for what it's worth, mine do the same thing. I don't have an opinion or enough experience to know if this is "correct" or not though.
But, from what i can tell these are really more of a wiper so thus far it hasn't concerned me all that much
As i understand it, the slides seal only when fully in or fully out, so the seals are the face mounted bead strip
They do sell slide seal lubricant, which seems to be nothing more than a dry silcone spray. Actually the one I bought goes on like a foam.
I tried it it on part of one of my slides. I couldn't really tell that it did anything, and further research at more active forums tell me that most experienced folks don't waste tehir time or money on the lube or treatments.