When a can electro in a chassis I was restoring broke down and leaked a brown, smelly oily liquid onto the table I thought I'd look it up to see if it was toxic or carcinogenic (like for example poly-chlorinated-byphenyls are).

I was relieved to find it is mainly borax and water. Nothing to worry about. Phew!

I basically knew how electrolytics work, but the Wiki entry was very educational. Remember the exploding Chinese electrolytic capacitors some years back? Here's the story behind it, pasted from the wiki:

"The Japanese manufacturer Rubycon was a leader in the development of new water-based electrolyte systems with enhanced conductivity in the late 1990s. The new series of non-solid capacitors with water-based electrolyte was called in the data sheets "Low-ESR", "Low-Impedance", "Ultra-Low-Impedance" or "High-Ripple Current" series.

A stolen recipe of such a water-based electrolyte, in which important stabilising substances were absent, led in the years 2000 to 2005 to the problem of mass-bursting capacitors in computers and power supplies, which became known under the term "Capacitor Plague". In these capacitors the water reacts quite aggressively and even violently with aluminum, accompanied by strong heat and gas development in the capacitor, and often leads to the explosion of the capacitor."

Heard that our bodies have electrolytes in the blood, but I think it is probably not the same stuff in capacitors...

An electrolyte is usually a simple chemical such as salt, potassium or calcium. Our bodies have heaps of it. I agree that the types found in condensers may be considered hostile by our immune systems.

It is to be noted that PCB's were used, even in paper caps, (self healing) quite early and oil filled caps, which are as bad as wax paper types for electrical leakage and should be treated with the same contempt. I recently had both oil leakage and electrical leakage in a Philco Frequency meter's can type caps: The seals had failed.

I have actually had a recent experience with caps from that era, after realising when a device was built. When tested they failed. Prior to that I had seen motherboards with the counterfeit caps & pretty much every electrolytic had an orange brown ooze atop them.

That was multi million dollar hit (Viz hundreds of millions) as the highest percentile of failures were within the boards warranty period.