IMHO:
You should be extreemly careful persuing an "oil change". If the xfrs are "50 yrs old" they probably are PCB contaminated, unless they were "cleaned" previously. They should have a tag indicating that they were "cleaned". I believe that this is a FEDERAL EPA (DEP) regulation.
THE PCB'S ARE A VERY SERIOUS MATTER. If you create a "spill" the clean-up costs can be astronomical. There used to be a test kit for a quick field analsys of xfr oil, but I don't recall who it was from.
BTW, If your "switches" are oil filled, be careful also, if they are old.
John
