I believe there is such a thing as an official MB sunroof slide paste, although it comes in a large container. My personal experience is that getting it squeeky clean, then applying a light treatment of silicon spray lubricant is better than using any type of grease, although I suspect that the same situation may not apply in all climates.
2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg
And remember folks, Only You Can Prevent Hilliary.