This Site Might Help You. RE: how lubricate a plastic zipper? plastic zipper repair
Yes candle wax or soap will lubricate a zipper - but if the problem is caused by dirt or grit clogging the zip or slider then all you are going to do is like glue it in, use a quality zip cleaner once a year to clean the zip, then wax. McNett Zip Care Liquid Zipper Cleaner Lubricant comes with a brush to clean between the bits of zip, available from Amazon
The Best Answer to use WD-40 is a terrible idea. Not only can the solution damage some materials (the answer even mentions the risk of staining), if the problem is dirt accumulation in the slider, a greasy liquid lubricant is just about the worst choice (aside from maybe fire). You want to clean the slider and use a dry lubricant as others have suggested. There is a product called Zip Care that is specifically made for this purpose.
Normally, most of the plastic made zipper elements are quite smooth enough. I supposed that must be the slider problem. You can spray WD-40 to the slider body seam. But please be carefu not to get stain to your clothing.
