I bought what MB recommends for lubing the sun roof. It's called Gleitpaste and cost me over $20 for 500 grams. p/n A 001 989 14 51
I use this stuff for many things besides the sun roof. It's kind of a miracle lubricant. I lube the sun roof twice a year.
I doubt that dealers use this lubricant (they're too cheap). The last time I talked to a dealer about it, I think he told me they used some kind of spray they get from Audi.