Since the oft referred to generic Mineral sucks on many levels and DI is right out that leaves using fluids made for electrical equipment cooling. eg
http://www.cooperindustries.com/content/dam/public/powersystems/resources/library/900_Fluids/CA900001EN.pdfLike the Shell Diala-AX we use in lasers it is reasonably priced, I believe for a 55gal drum of Diala we pay $750 USD. Biggest issue with Diala is that it strips the polymers out of many plastics - especially PVC as in what most PCIe power wires are covered in. PVC starts going rock-hard in weeks. All plumbing must be metal or a plastic/rubber that will not be attacked. Same applies to using Mineral oil but it takes longer. In every fluid or oil I've come across any labels or stickers will soon be floating around...
Acrylic eg Plexiglass(tm), polycarbonate eg Lexan(tm), Nylon, Polyethylene, and fiberglass/epoxies all hold up with no issues.
Might want to look up the mfgr specs on the large can capacitors used to see what their seals are made of. If silicon rubber, most likely ok
unless silicon fluids are used. If is a different rubber compound need to verify chemical compatibility of it with the fluid used. The other thing that need investigating: All
thin engineered dialectic fluids are designed as penetrating oils to get into the tightest places. Experience proves that o-rings do not stop it so not sure if it can affect a 'solid' electrolyte can cap.
Usually near water thin so easy to pump, good heat capacity and flows well over the heat sinks.
As a viscosity reference, water is 0.890 cP
edit: Hmm. This looks rather interesting
http://www.clearcoproducts.com/dielectric-silicone-fluids-sto-50.html and
http://www.clearcoproducts.com/pdf/dielectric-fluids/Dielectric_Properties_Pure_Silicone_Fluids.pdf"inert to all substrates" Available in 5gal pails or 55gal drums.
I may get a 5gal pail to um, investigate using something other than Diala (very restricted shipping regs, hardens the laser cooling lines). Yeah, thass it. To improve ease of laser maintenance...