Is there any particular reason certain oils are used? Is because of corrosion issues? Or even humidity issues?
Some oils spoil or are corrosive because of contamination. Other factors are heat transfer and high-resistance insulation.
Before I retired, we used Shell Diala
TM transformer oil for insulating high-voltage supplies.
So far as I could tell, it was basically pure motor oil, with BHT (or similar) added to retard oxidation.
Butylated hydroxytoluene is an anti-oxidant, whose use in food is controversial.
(For high-voltage purposes, it was necessary to pump a vacuum on top of it to remove bubbles.)