Oil for welding transformer tank

[Tedzed1]

For a high voltage transformer, working 24/7 at high temperatures, the stuff they use will be expensive. But for a step down transformer, only used now and then, I wonder what would be "adequate for purpose" and a good deal cheaper?

I was wondering if the same solution could be applied to this as some TV LOPT's, such as in a Murphy V310, the LOPT can is filled with oil,
and most of them can be refilled with vegetable oil, but NOT mineral oil.

Just a suggestion.