When you buy a better quality foil capacitor, it could be that the manufacturer added a warning Not because you should watch what you spend but a warning how it should be polarized. I received it with the Auricap-R capacitors that have a long and a shorter wire. The long lead is the inside foil and should be treated the same as the red lead on the standard axial auricaps.
The outside foil is a useful noise shield when input and output impedances are considerably different values. This is true when the outside foil is connected to the lower impedance (sometimes earth) .
Coupling / X-over caps
Circuit outputs are always of lower impedance then inputs and should be connected to the outside foil.
Test with scope
In the little movie I made, you will see several capacitors going through my hands. Wima capacitors, the auricap capacitor and at the end 2 Philips (Mullard) mustard caps. You will see the difference in amplitude on the scope. I was surprised there was a lot of difference in sensitivity between the different brands and form factors. The big foil caps used in the stabilized power supply did not show any differences.
The oscilloscope was measuring at 5mV/dev most of the time so it was hard to keep a steady waveform. Still I hope you get the message: Even though foil capacitors are non polarized, it can matter how you install them.
In week_47 issue: negative feedback, cure worse then the disease?