There is discussion of restoration materials, namely re-blacking and waxes, in Fountain Pen Geeks. I would continue posting there, but what I have to say on the subject is (as you might expect) a little long-winded for a board comment.
As a restorer, I believe that one should do the minimum necessary to a pen to bring it to good working condition and an acceptable appearance. If you want old pens that look like they came from the factory yesterday, try another restorer. I don’t do that kind of work.
Re-blacking is irreversible, no matter which method you use. Even Syd Saperstein’s Potion No 9, which does not remove any material, cannot be fully reversed. It will wash off smooth surfaces, but removing it from chasing and the marks a pen acquires over decades is very difficult or actually impossible. Methods that depend on removing the oxidised layer will also remove detail of chasing and imprints. Then, of course, it will oxidise again. So what do you do? Remove more surface layers?
If you have a common and inexpensive old hard rubber pen that has turned absolutely yellow with oxidisation, and you have no intention of selling it, then it seems to me that there is no great harm in using the latest product to come on the market to re-black it. It apparently removes very little material and it may make your pen more appealing to you for a time. It may fade again. After all, some black hard rubbers are more prone to fading than others. It may, for all we know, dissolve into a puddle of goo some way down the road. We don’t know the long-term effects of any of the current products. By time we do it may be too late.
My strongest word of caution is: don’t re-black uncommon and/or expensive pens. We have a duty of custodianship to our pens and to the hobby. Old pens are important historical artefacts and should be treated as such. That’s not to say don’t write with them, or keep them in a glass case but it seems sensible not to coat them with chemicals whose ultimate effect we don’t know. Black hard rubber is very durable and, left alone or conserved with the minimum of intervention, these pens will last a very long time. Using inappropriate substances on them may, in the long term, destroy them.
Most oxidisation is not unattractive. Careful hand polishing will make most hard rubber pens look good. That’s all that I do with the pens that pass through my hands.
I don’t have much to say about the use of waxes. I don’t believe they have a place in pen restoration. All, so far as I am aware, have been shown to contain chemicals that are, at the very least, dubious. Some attack metals. All seal the surface of the pen which doesn’t seem all that clever a thing to do. They are very, very hard to remove. They produce a wholly unnecessary gloss. Why do it?
Over the last decade I have seen the supply of old pens begin to reduce. Nobody is making any more old pens! We must look after those we have and that includes using a minimum of chemicals and only those we know to be trustworthy.