Not sure about the first, I've always wanted to know as well as when you own a classic car door seals aren't exactly easy to come by and so many restored cars have them, I don't know how they do it.
The second part I can answer, the best way to protect the plastics, rubbers around the car is to clean them regularly and give them a wipe with some form of plastic conditioner, anything petroleum based that helps keep the oils in the plastic. Stay away from ArmorAll however, it doesn't tend to last as long I've found and have heard (only heard) that it's because ArmorAll brings the oils in the plastic to the surface (to keep them shiny) rather than actually add or protect them.
Good plastic conditioner will do all of that, including sun protection which helps to keep plastics looking good but I don't know just how long you can do that for - whether it will indefinitely is another thing.
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