I've only known BMW Gummi Phlege to be available through BMW dealerships. Hopefully there is one near you, because it really is an excellent product. :up
If I'm looking to soften the weatherstripping, I'll use the Gummi Phlege. If I'm looking to dress it for a car show, them I apply 303 Aerospace Protectant.
I don't allow the weatherstripping to get dirty on either the daily driver or the Vette (pet peeve of mine), but if it were dirty I would clean with a mild soap and water, rinse with a separate mid-grade microfiber towel/bucket. Dry the weatherstripping by hand and then also allow it to completely air dry on it's own - then apply the Gummi Phlege, gently working it into the seals. Allow the Gummi Phlege to sit for at least 10 minutes and then gently buff off the excess. The Gummi Phlege both softens and imparts a very nice richness to the weatherstripping.
Maintain with the 303 and it will look great. :xyxthumbs