Your local power company has it. Generally at their transformer maintenance
shop.... I got 2 gallons which will last me a lifetime free. They buy it by
the rail tanker and said the cost to figure out what to charge for 1 - 2
gals was more then it was worth grin. They use the stuff in transformers.
It's highly refined mineral oil and has anti oxidation additives etc. so
it's not "just" mineral oil. There are several makers and brand names but
the stuff I got is made by Shell and is called Shell Diala (apparently named
ala dialectric oil).

Mike, W9WIS