BMW used to make/package a glycerin-based rubber care product in a tube that was a gel that would liquefy when applied. Neat stuff.
But, since it was discontinued, the next best thing I've found is nextzett's rubber care product
. The applicator sucks, but the product is good.
I have 303 and Sil-Glide on hand, and have used #40, but for this particular task, none have been as good, especially in keeping old rubber pliable.
Also, when allowed to dry properly after application, they leave little to no residue, so the edges of frameless windows don't get smudged.
But, still wish the original BMW Gummi-Pflege was still available.