Let's just bear in mind that rubber, most plastics, silicones, and other similar compounds are by nature non-porous. That's exactly why they are used for seals, O-rings, boots, and gaskets, etc, anywhere you don't want fluids (including gaseous fluids like air or vapors) to leak out.
The failure mode for many of these materials is that with time, heat, cold, UV exposure, ozone exposure, mechanical stress, combustion byproducts and chemical exposure, vibration, friction, and the like, the materials breakdown, outgass, and lose some of their embedded plasticizers and elasticizers, the very compounds that make them so good at what they do -- stay flexible and seal things up.
Once the material is compromised, there is no restoring the original compounds that are gone. Now there are some products that can seep into the spaces vacated by those compounds -- and they work by softening the surface and causing the rubber to swell and temporarily re-seal the shaft or whatever the function was. But those are not permanent fixes and the material continues to degrade, and some of the additives (because they are acidic in PH) actually accelerate that process. The product itself eventually gets displaced by the same process as the original degradation. Eventually the material is too compromised to help by any means.
We've all seen brittle, cracked seals and boots and failed O-rings and gaskets. Would a product have forestalled the failure? -- maybe -- but not for long. If there was an effective additive that really "fixed" rubber, don't you think it would already be in every fluid that contacts a seal?