NavList:

There are perhaps others in this group who might direct you on the best plastic sextant lubes, but my experiments started initially with two things, both of which appeared to work very well.

 The first I was sure was safe for plastic: zipper lube from west marine, a silicone compound that I keep on my winter enclosure zippers on the boat, which are plastic.  Its a nearly clear silicone coumpound in a tube. I applied it sparingly with a small modelers paint brush at key friciton points. It seems to maintain consistant performance in the temps I sight in.

The other i have tried is my precison oiler with musical instrument oil as it was on hand and I knew what it was meant to do.... (available at any instrument store that sells woodwinds..brass, etc.)..  I play a beautiful handmade proffessional flute, and the musical instrument oil is a very pure clear light low viscosity oil, that maximizes the key speed and reduces key/valve potential sticking and friction in the mechanism. It doesn't have any solvent or distillate additives I know of to soften plastic etc...and its very light viscosity allows it to flow in microlayers via capillary action into places zipper lube can't get, without thickening or getting gummy later.

I know of no ill effects so far, and they both seemed to help the sticking on the plastic sextant.  The old adage about use sparingly seems to work well. The precison oiler for flute axles is really good on this count. Tiny flow on contact form a precison tip.

I first got an inpiration on the plastic sticking, flexing the frame a little, then moving agian, having handled plastic nuts and bolts and plastic piping....which are famous for all of a sudden binding. Lubrication seemed to increase accuracy several tenths in setting up mirrors and index correction.  I also learned to alter my grip to a lighter finger grip instead of my large strong paws grabbing it full hand. 

I have read that pure Jojoba oil is the best substitute for the old whale oil that was once used for sextants. Its use on plastic I have no knowledge. THe other that comes to mind is high grade sewing machine oils for such as Bernina.  I knew a sewing machine repairman who was cultlike in his insistance on the use of their own highly purified oils.  He insisted that cheaper grades would leave harmful residues or degrade. Pure gun oil as well, also light viscosity to prevent drag at low temps.  

I have shied away from anything in a spray can with a distillate or solvent carrier as some things might attack plastic?  I otherwise have a love affair with Boeshield T-9 on my boat for my outriggers, pot hauler, engine, and many other items, etc...but so far was reluctant to use something with a carrier and corrosion residue that serves no purpose on plastic. The residue I believe is parafin base and probobly harmless.  

MC