I use Gummi Pflege (rubber care is literal translation) on mine as part of my servicing, when ever I do my king pins, I treat all my rubber seals with it after cleaning out any debris, it conditions the rubber making it soft and plump, there by it has a better chance of creating an effective seal.
Never let seals get stiff as they'll end up with a memory effect and stay the same shape and not springing back.
