I like the 303 Aerospace protectant. You can often find it at boat dealer showrooms. Here is a blurb from the Classic Motoring Accessories ( http://www.properautocare.com ) website:
Maybe not a surprise, but no longer are a car's rubber seals made from `rubber'. To keep the inside of your car quiet and dry, car makers use a specialized synthetic material called EPDM, (ethylene propylene diene monomer).
Real rubber, or blends containing real rubber, just cannot endure the direct exposure to sunlight (ultraviolet light) and the harmful-to-rubber oxidizing gases in our atmosphere . To quote from the engineering specs, EPDM is "Ideal for outdoor applications because of its excellent resistance to ultraviolet light, ozone, oxidants, and severe weather conditions".
EPDM is great stuff, but it has some downsides. It's tear resistance is only fair, so sticking & tearing is not uncommon. And just like real rubber, it has extremely poor resistance to solvents (petroleum distillates) and oils. "You mean I don't have to `protect' my car's door & trunk seals?" That's right, and treating your seals with a leading-brand "protectant" is almost always exactly the WRONG thing to do to your seals. Any chemical product that contains oils or petroleum distillates is incompatible with EPDM.
TIP: NEVER apply any rubber or vinyl treatment that has an oily or greasy nature or contains petroleum distillates. READ THE LABEL! If a product contains petroleum distillates, do not apply it to your rubber seals.
Though its powerful UV screening benefit is not needed in this application, 303 Aerospace Protectant is great for cleaning EPDM seals and to keep them clean. 303 Aerospace Protectant makes EPDM seals look like new and PREVENTS sticking and tearing. Because it is safe for EPDM and prevents sticking & tearing, manufacturers exclusively recommend 303 Aerospace Protectant for this application.