I use the Zipper Lubricant Stick made by GearAid to lube the plastic zipper on my drysuit. Works great to keep it moving freely. If you get sand or something in the zipper just use a clean soft toothbrush and then reapply the lube.
It is a soft-wax semi-solid base that they claim also helps protect the zipper against corrosion, UV damage, water, and extreme temperatures.
In response to your point about using silicone spray, it interestingly specifically advertises that it is silicone free. Although I couldn't tell you why silicone is bad, if it even is bad, for the zipper.
This brand is also sold by Kokatat, arguably the premier kayaking drysuit manufacturer, so you can view that as whatever level of endorsement of the product by them that you choose.