Mercedes packages something called Gleitpaste for the sunroof but it's very expensive. P/N A 001 989 14 51. I think I paid about $22 for a 500g can but that was quite some time ago.
On the other hand it's really great stuff for more than sunroof maintenance. I use it for everything from sliding doorwalls to door hinges. One can will last a lifetime.
I doubt dealers use this stuff. One Mercedes dealer told me they use a spray with an Audi part number. You might call your local MB (or any German car) dealer and ask them what they use.
Don't let the sunroof go too long w/o maintenance or you might wind up with expensive repairs. I lube mine twice a year.