Mineral Oil (available at your local drug store).

Just be prepared for some strange looks when you purchase a gallon of
mineral oil all at once. :-O

Is motor oil a conductor?

Might burn and sludge up your resistor. Lots of additives you just don't
need.

I had a cantenna for 20 years with light mineral oil. there was a little
seepage to the top of the lid through the vent. I actually had transformer
oil but never used it because of the thought of that stuff seeping. It
really is a lot cleaner. Check feed stores, Vet supply stores, paint
stores, hardware. Check this out:
http://www.pugetsystems.com/submerged.php