1981 300SD & 1979 240D power sunroof problem

I have both cars and they both share the same problem, worse on the 300SD. The power sunroof opens very slowly and closes slower, and getting even slower with time. We're talking slow! I'm almost afraid
to open them now for fear they won't close again. I have no experience with sunroofs previously so I don't know where to start or what this might be a symptom of. Failing motor? Lack of lubrication somewhere? I did try WD40 on the slide rails that I could reach.
Thanks for any help or direction.
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MB makes a special lubricant for the sunroof. It is a paste in a can titled "Sliding jaw grease". IT works. I have tried all kinds of lubes but nothing does the trick like the correct stuff.

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