Good! Don't give up yet!
Googling, I found that Texaco (now Chevron/Texaco) makes a transformer cooling oil:
http://library.cbest.chevron.com/lub...56c410059c747/$FILE/IO-22240.pdf
IT has a kinematic viscosity of 8.8 centistokes, at 40 deg C, and changes to 2.3 at 100 deg C.
According to the Electric Energy Society of Australia, mineral oil is now used, replacing the old PCB (polychlorinated byphenyls) oils, which turned out to be highly carcinogenic.
http://www.aeema.asn.au/conf2002/zip...slav_Kovar.pdf