Autozone and the like sell spray protectants for use on constant velocity joint "rubber boots". CV joint boots normally split over time rather than degenerate. But perhaps these boots are not rubber as they have to contend with the constant presence of mineral ois.
Are you saying some parts have deteriorated / failed, and now you have some new ones which you want to protect?
There are many different sorts of rubber. Some are prone to attack by UV, if they see sunlight. Others by ozone. Sometimes rubbers go soft and sticky for little apparent reason, with no consistency between nominally identical parts.
It seems to me that your mysterious parts are deteriorating they are probably made from the wrong material.
You seem to think that silicone spray might behave like "leather dressing". But that works by soaking into the fibrous structure of leather. It is likely to make rubber swell or soften, but may help it stay flexible.
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