Composition of sticky gel in Film Capacitors
It's capacitor gel. Electrolyte. Here are typical capacitor safety sheets:

http://www.ioxus.com/files/8714/3681/1244/Ultracapacitor-MSDS-02-06-2013.pdf

http://www.rpiparts.com/ds/CMC042msds.pdf

Note the ingrediants: Boric Acid/Ammonia, and Tetraethyl-Ammonium-Tetrafluoroborate/Acetonitrile.

There is no way of guessing what the electrolyte in a capacitor contained. You have to find out what the manufacturer was using at that time.