I researched this subject a few years ago when I bought the 968 cab, there's lots of info on VW Eos forums (Eos roofs are notorious for leaking). Krytox is manufacturers treatment of choice but is damned expensive. Forum view was Gummipflege was as good and about 1/4 the price. I've used it on both cars for about 4 years now, no complaints from me!!
There is also MB's proper stuff (A 000 989 60 60 special sliding compound, also described as seal/mold lubricant at OEMpartsdirect.com or A 001 989 50 51 lubricant for seals, also described as grease at OEMPartsdirect.com - I think they are the same thing) but it really is expensive but it lasts for ages as you do only use it very spareingly