Sorry, you didn't tag me in so I had no idea you'd asked me a question. (Still not 100% sure, but will answer anyway.)
The Gummi Pflege (actually German for "Rubber Care") does just that. It seems to keep your rubbers soft and flexible. I love it, but it's really not a cosmetic treatment.
The 303 Protectant is mainly a UV protectant and has a slight "Back to Black(TM)" effect. It can be used on rubbers but is best on plastics, IMO. Spray on, wipe off to a dry sheen.
It's a penetrating oil, not a lubricant and certainly not good for rubber, though it will make your rubber look dark and shiny.
For a while at least. I wouldn't use the stuff, but I keep my cars for ages.
However there are plenty of videos on YouTube of people advocating you use WD40 to revitalise the rubbers on your car and it's your car, so if that works for you, go for it.
You could probably use actual vegetable oil and give your rubbers that "just new look". (I expect there're some TikToK videos showing just that.)
https://en.wikipedia.org/wiki/WD-40