Best grease for sunroof

Hello,

I was planning to clean and re-lubricate the guide rails of my sunroof this weekend.
Now, according to the well-known DIY manuals, you absolutely must not use "ordinary" grease, but rather a very specific type of grease from VW.
Why is it not allowed to use normal, water-resistant fat?
What is so special about this grease from VW?

It will be silicone-based, so it won't stain the ceiling.