1/8", which I think is about .090 weed wacker line will work, I have used it on an old dodge ram that had hardened rubber window trim. If the rubber is soft, I will use something a little thicker but the weed wacker line worked for me.

Whatever you don avoid vaseline, it can destroy the rubber- you may want to check on an International forum to see if anybody has replaced their gaskets with generic rubber- it comes in rolls in different profiles and is fairly cheap- it will make a world of difference