Q: How do I lubricate a stiff metal zipper?
First clean, then lubricate. Brush the teeth with a soft toothbrush and remove lint, sand and dried detergent, because adding lubricant over grit produces a paste that makes the slider drag more. Work the slider along the full length a few times to distribute whatever is left, then wipe the tape with a damp cloth.
For lubrication, paraffin wax, a silicone stick or a graphite pencil run along the teeth are the safest household options. Apply a thin film to both sides of the chain, then cycle the slider ten to twenty times and wipe off the excess so it does not attract dust. Avoid cooking oil, grease and heavy petroleum products: they oxidize, stain the tape and hold grit.
If the zipper is still stiff after cleaning and light lubrication, the cause is mechanical, not friction. Check for a bent tooth, a spread slider or a tape edge caught in the channel. Lubricant cannot fix a worn slider, and using it to mask the problem usually leads to a failure at an inconvenient moment.