Easy test if that cap is failing is to look at the seal between the terminal and the case. If glass, do not bother till it rusts apart, if it is rubber then beware of any liquid visible there as the rubber degrades.
There are still plenty of power transformers in use filled with PCB oil. Because the regular oil change equipment that takes the old oil out, filters and dewaters it is not cleaned between uses this means all of the transformers ( even brand new ones) will have some small volume of PCB oil contaminant in them, and older ones ( typical age is 60 years, simply because they do last so long in service) will never get all the oil in the paper insulation out ever. Then look at all the older PFC capacitors in use in utility switching yards, and still in use, full of PCB containing oil. Then look around for 40 year old wire, if it is oozing a green liquid that is also PCB based plasticiser in the PVC wire sheath, reacting slowly with the copper wire. I work with it daily, plenty of it around all over the world.