They're two different things. First you have your "rubber cleaner",
which as you say is all nasty solvents, to get off the greasy ink and
grime. That workd just swell.

But in getting out the grease, the solvents also leach out the
"plasticisers" out of the rubber. They're the additives that make the
rubber soft and pliable. You lose those and the rubber gets too hard.


The oil of xxx things are "rubber rejuvenators".... a poor man's
plasticizer. These soften up the rubber back to its original
condition.

Both have their uses. After you use the first ones a lot, you probably
will need the second ones.