Plastic-on-plastic lubricant that wont leave a residue?
Polydimethylsiloxane meets almost all your requirements. PDMS is non-conductive, optically clear, inert, non toxic, non flammable, and extremely slippery. It's typically an ingredient in silicone oil, but you can probably buy it separately somewhere too. 

Other than PDMS, mineral oil would also work. Much easier to get too. 

There will definitely be reside, if you want to call it that though. Neither substance will evaporate, so it's going to be there forever. On the other hand, that property will help you disassemble them if you need to repair it in the future. 

Maybe just use as little as possible? Probably just on the contact surfaces.