As previously mentioned by other members, the original oil in the Pot[s] may contain PCB - therefore becoming a Hazardous material [like Asbestos (ACM) abatement].
I say "May" only because maybe the original oil was previously changed to a Non-PCB type??? [not an easy thing to verify looking at the oil!].
This is added just as a "BTW" thing-ee
I would imagine that changing the oil is something that would not only be a good idea - due to possible dielectric breakdown from age, the Lightning surge, and since you are doing some "R&R" on the coils -, but also would be mandatory replacement with non-PCB materials since it's being "Exposed".
Sounds like an "Icky, Sticky" job!
On the subject of the 2 phase systems:
PLEASE, PLEASE, PLEASE!!! get some pictures of active systems [open delta Tee and the "full" three xformer systems, if any], along with the one you are servicing.
Man, I would love to add those images to the Tech Center's archives!!!
Send them to me for posting.