I have a 1970's backhoe which means old seals abound and leaks aplenty.
I have had amazing luck with ATP205 in my cars and refreshing general rubber materials like door/sunroof seals and gaskets.
I've submerged rock hard old seals in a cup and woken up to a soft almost fresh feeling seal the next day.

What I have come across on here though is minimal posts about this wonder fluid. The primary ingredient is Diethylene Glycol Monoethyl Ether.

https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/2816155/atp-205-reseal
That post mentions it eats plastic/teflon/delrin type seals.

How corrosive and dangerous is it to them?
Hydraulic cylinders have teflon ring seals in them. I want to make sure I'm not creating more problems than I fix but a plasticizer could likely go a long way on such old seals.