I've recently discovered Meguiar's G15812EU Ultimate Black Plastic and Trim Restorer which, I have to say, is actually very good. I used it on my old A4 before it was handed over to the new owner around 4 months ago, and the plastic still looks great. This product is also for use on rubber, but I'm not overly fussed about looks, more just preventing degradation issues as they age (which this product likely does anyway).

However, does anyone care to much about treating the window seals, and what do you use?

I've since used this:

Autoglym VRC500US Vinyl & Rubber Care, 500ml https://amzn.eu/d/2LTVxQJ

Very easy to use; all seals done in around 10 minutes. And I'm really pleased with the outcome.

I’ve been using this for a while: Gummi Pflege is the ideal Summer and Winter protection for your cars rubber. Apply to rubber seals and prevent frost, UV damage, and cracking which will help your door and window seals in prime condition.

 

https://youtu.be/1TXP2F4CDrA

Might have to look at using some of this on my A6 rubbers.