My daughters VW EOS is very prone to roof leaks if the rubber is not properly treated. There is a lot of info on a product called
Krytox, by Dupont. I have some, conditions the rubber nicely, and turns it nice and black, though the liquid is clear. I would recommend this stuff.
There is also a product called
Gummi Pflege. Also has high marks.
Then there are always the products from
3M.
Lots of options.
L