You can use a shoe polish sponge, the type that has silicone impregnated (clear version) that will bring the rubber up like new and repel water (stops door rubber being damaged in freezing weather, when the seal freezes to the door)
I don't exactly know what's in mine, just that it was the only product recommended by Mercedes to use on the folding roof rubbers on my SLC , without it the rubber stuck to the glass & squeaked.