For ordinary dust and dirt, use lukewarm water with a drop or two of PhotoFlo.
Submerge the film and wipe, lightly, with a clean Q-Tip. Hang the film to dry in a dust-free area.

For more aggressive cleaning, isopropyl alcohol will work. Use the "91%" variety from the drug store.
Even if you buy "pure" alcohol, it will absorb water out of the air, anyway.

After that, PEC-12, as mentioned above will do a good job if you follow the instructions.

If none of that works, the only things left are carbon tetrachloride or trichloroethane but you don't want to mess with that stuff. Even if you can find them, stuff is toxic. (Harmful vapors.)