"Poly" just means synthetic polymer. Film or Foil? Foil capacitors are made with metal foils (usually aluminium) separated by layer of dielectric while Film capacitors are made with metallized layers on dielectric films. Film capacitors are smaller but Foil capacitors offer larger current capabilities. Polypropylene capacitors have lower dielectric constant and better high frequency characteristics, but are larger and more expensive than polyester. Any Poly capacitor will be better than electrolytic (but much larger). Poly caps are non-polarized, but one side is usually marked with a bar. This is the end of the outer layer. Connecting this side to GND provides shielding.
Capacitor upgrade jargon definitions
What is "poly" ? -STYRENE or -PROPYLENE? Which is available as the least cost replacement for bipolar electrolytic radials, 100V 8 and 15uf ? Are "poly" caps foil, or film, or foil AND film construction? People use "poly" and "film" interchangeably as though they are the same thing. Are they? And lastly, are "poly" caps non-/bi-polar ? If not, how do you know which way they go in? Thank you.