Modern dielectric oil (aka transformer oil) is refined mineral oil
with an antioxidant (tocopherol, BHA, or BHT) added to retard
spoiling.

Shell Diala AX
ExxonMobil Univolt 65

are the two major kinds (there's one from Castrol,too)

Both are sold (in 5 gallon pails, typically, as a minimum quantity) by
"jobbers" which can be found on the mfr's website (you enter a zip
code and gives you the distributors within X miles), or by looking in
the yellow pages under Oil,Lubricants-Jobbers. Used to be in the $4-5/
gallon range, but I just was talking to someone who had to pay around
$50 for a 5gallon pail. (probably a hangover from $100/bbl crude
prices)

You *can* use USP White Mineral Oil (laxative) available in pints at
the drugstore, gallons at the feedstore (If you've got a colicky
horse, gallons are the quantity wanted), but it's
a) more expensive
b) not water content controlled

For HV dielectric purposes water content (in the ppm range) is
important. So is particulate contamination. For a dummy load,
probably not so much.

Another inexpensive source of mineral oil without many additives is
hydraulic oil (as used in, say, tractors, etc.).

Even "straight weight" motor oil without additives/detergents can work
(look for the SAE 10,15, or 20 viscosities).. it can be VERY cheap on
sale as a "loss leader" to get folks into the store (since nobody in
their right mind would actually run this in an engine)

For a dummy load, viscosity IS important, because convective flow is
important. (viscosity change with temperature, too...)

Silicone would be massive overkill, and we won't even get into
Fluorinert. BTW, if you spill mineral oil, it cleans up nicely with
detergent and water.. the same cannot be said of silicone or FC-xx..
Silicone oils are almost impossible to remove.

Jim, w6rmk