I now use
Gummi Pflege
used to use a silicon spray.

Both are good, but the Gummi Pflege is easier/less messy to use and isn't silicon based.

When I used the silicon spray I used to spray it into the cap lid and used a small bit of sponge to apply it

( trying to keep the silicon off the paintwork/windows) the Gummi Pflege has a built in applicator.

I normally redo the seals a couple of times a year to keep them soft.
