Cost: About 7 bucks.
Whenever I mention this stuff, I am surprised how many look at me like English is my second language. However, the fact that some are supposed hard-core bimmer enthusiasts is most surprising.
Gummi-Pflege (82 149 407 015) - which translates roughly to "rubber maintenance" - is BMW's solution for exactly that.
Its been a staple in the Z3 community where the rubber seal that runs between canvas top and window also encloses a water drain channel. To accommodate the folding of the top, the seal is split in a couple places along the window. When the rubber dries or becomes weathered, it tends to shrink. When this occurs in the Z3 window seal, the water draining in the channel leaks at the splits, and the driver develops a nice puddle on his/her lap.
For older cars like my 635CSi, Gummi-Pflege helps recondition older seals and reduce or stop water intrusion in typical problem areas such as the trunk and window seal. It certainly makes more sense to spend $7 and a little time trying to recondition a leaking seal or one that propagates wind noise before looking to replace, say, a $300 door seal.
The official description from the tube:
"Protects and takes care of rubber items. Prevents e.g. door weatherstrips from freezing up in winter. Maintains tubing connectors pliable in engine compartment. Apply paste evenly and sparingly on clean, dried surfaces."
As the description on the tube suggests, this stuff will work on most any rubber. Engine bay tubing and connector boots, door or window weather seals, etc.
Gummi-Pflege is not intended to be a "blackener" like Black Opal or Forever Black. Although applying Gummi-Pflege will restore some of the color, the other beneficial results of its use will last beyond the loss of the restored color. If the rubber still looks unsatisfactory after I am finished applying the Gummi-Pflege, I'll simply apply a little Black Opal for color.
You won't need to use much. This 75ml tube should last a long time even if you applied it to all the rubber in all your cars and throughout your house.
NOTE: It is probably not a good idea to leave the tube in the car. It is possible the Gummi-Pflege may liquify in the heat of a car and leak from the tube.
Gummi-Pflege looks like an ointment. The tube is topped with a foam applicator, but the best way to apply Gummi-Pflege is with your fingers. This serves three purposes. First, the heat from your fingers melts the Gummi-Pflege, and it seems to soak into the rubber better. Secondly, you will need to use your fingers to get the Gummi-Pflege in, around, and underneath the odd shaped seals a BMW can have. Thirdly, you will be able to feel when enough Gummi-Pflege has been applied. Just pay attention to how the rubber feels when you are applying it, and how it feels after, perhaps, even a third application. You will be able to tell areas of seals that need additional application from those that are as conditioned as possible with Gummi-Plege, simply by running your finger tips over the seal.
When reconditioning primary weather seals in particular, pay attention to corners, bends and other areas where the rubber will already be thinner. These areas will be the initial points for water intrusion and rubber degradation. Extra applications are always warranted in these areas. Reconditioning the seal will also give you the opportunity to inspect the seals for condition and fit (see below) that may contribute to water intrusion or wind noise.
For maintenance of seals that are in good condition, simply wipe the rubber down with a wet sponge or rag until clean. Allow to dry, and apply the Gummi-Pflege. One application will probably be all that is needed if its regularly maintained or not subjected to harsh conditions.
For restoration of primary weather seals and such, clean well with mild soapy water (Dawn works well). For problem areas, light scrubbing with an abrasive sponge will usually be all that is needed. Rinse clean and allow to dry. Thoroughly apply Gummi-Pflege and allow it to soak in (it only takes a bit). Run your fingers along the rubber, and you will be able to feel the differing areas of conditioning. Re-apply as necessary for the rubber to feel uniformly conditioned. After a bit, the rubber will no longer absorb the Gummi-Pflege, and you may need to wipe up any excess. Gummi-Pflege isn't harmful to paint, but you probably don't want it to stay there...don't worry about painted surfaces while applying (though, you might want to keep it off any cloth); simply wipe off any that spreads to a painted surface after you're done.