I would say try brake fluid but, I haven't experimented with it yet.
I know some truckers that swear by brake fluid for restoring weather checking or mild dry rot on tires. One of them told me to mix it 50/50 with WD-40 and apply daily until results are achieved. He also asked me if I've ever seen a master cylinder rubber diaphragm in bad shape, I said no, he said because rubber loves brake fluid.
Since brake fluid is bad for paint maybe it'll work for you.
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