Since I went through this and did some research a few years back I'll
jump in here.
Most motor oils, including some so called "synthetics", and transmission
fluids that I checked on, use sulfur compounds as additives. Sulfur will
attack silver and silver compounds and dissolve them which has the
tendency to make the connections on the resistors in the dummy loads go
open circuit eventually. Manufacturers MSDS sheets are a good place to
start checking for sulfur.
Most any thing that does not have sulfur or other compounds that attack
silver or copper can be used. The difference is the coolants ability to
move the heat away from the resistor.
"Transformer oil" is specially formulated (without any nasty compounds)
to not only conduct heat efficiently but also to have a very high flash
point and high dielectric strength. It is the best choice for dummy
loads if you can get it. Most petroleum product distributors can get it
for you if they don't stock it. The problem for us hams is that the
usual container size is a 55 gallon drum!
Pure mineral oil is a good substitute if you can find it. It does not
move heat as efficiently as transformer oil and it has a much lower
flash point but usually it don't matter for a dummy load. I would be
cautious about mineral oil used for livestock. If it's really pure, go
for it, but I suspect that stuff sold in feed stores may have more in it
than just mineral oil.
For people who work in the railroad industry life might be easy. It
turns out that diesel locomotive engines use silver in the bearings so
locomotive oil is formulated with no harmful (to us) additives.
Unfortunately, it is usually not available in low viscosities or small
container sizes.
It turns out that aircraft reciprocating engines typically require a
"break-in" period that uses a non-sulfur based oil. If you can find it
at a distributer or an aviation service shop, it can be had in quart
containers and low viscosities.
I used Shell Aviation break-in oil (30 weight) in my rebuilt Bird load
because that was the only one of the above I could obtain locally at a
reasonable cost. It's been in the load for more than 10 years now with
no discernible degradation of the silver contacts.
73, Larry
Larry - W7IUV
DN07dg
http://w7iuv.com
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