How to lubricate a Lowepro zip?

I have a Orion AW and a toploader 70 AW and on both the zips are so stiff that its getting very difficult to open and close them without applying so much pressure as to risk damaging them.

Googling around, there are several suggestions for lubricating zips, but i am not sure how appropriate they are to camera gear, or if they work.  e.g.

1. rub zip with graphite pencil.  Will this produce harmful clay/graphite dust?  How do you tell if a pencil is graphite?
2. rub zip with dry bar of soap.
3. Rub zip with a dry candle

Any ideas?

Thanks!