If you make your own capacitors, you may be interested in what is a good oil to
use in them.  Several of the big oil manufacturers make good transformer oils
that are also used in capacitors.  Here are a couple of them.

Exxon Univolt 60, contains no anti oxidant additives
Exxon Univolt N61, contains anti oxidant additives
Shell Diala A, contains no anti oxidant additives
Shell Diala AX, contains anti oxidant additives

These oils are very thin, so they will penetrate between plates very readily.
These oils are also hygroscopic, which means it absorbs water easily.  So great
care must be taken to keep the container closed at all times.

Most people only buy the anti oxidant variety, for obvious reasons.  The cost
varies depending upon the supplier.  The minumum quantity that can be purchased
is 5 gallon pails.  The cost I have been quoted is $44.44 per pail for the Diala
AX and $32.40 for the Univolt N61.

Check with your local distributors for availability.

Other oils can be used.  The main things that should be considered are
viscosity, additives and clenliness.  The weight should be SAE10, ISO 32, or
less.  If it has a higher viscosity, it won't penetrate the plates well and the
air has a harder time escaping.  It should contain no additives, except for non
metallic anti oxidant additives.  The cleanliness should be ISO 17/14 or below.
The cleanliness level is the most difficult thing to determine.  If you work in
hydraulics, such as I do, you will have no problem analysing your own oil for
contaminants.  If you dont, try to buy your oil in sealed PLASTIC containers
from the factory.  Unless you are using a 3 micron or smaller filtering system
to pump your oil from the barrel, do not get your oil from a 55 gallon drum.  It
has a higher probability of contamination.

I am supplying this information for those that want to know what commercial
manufacturers of capacitors use.  I have looked into this because of the time I
have spent making a few homebrew capacitors.  I didn't want to get them finished
to only have them fail because I used an improper oil.

One word of warning on oils.  Don't use any oil that is marked "AW", which means
anti wear.  Anti wear oils contain zinc and phosphorous additives.  Zinc reduces
the oils dielectric constant.  So, it is possible that AW oil will cause a
capacitor failure.  Hydraulic oil is a thin oil that contains these additives.

That's my 2 cents on capacitor oils.

Scott Myers