I have a 1990 with the single reflector lenses. I noticed the headlights are aimed horribly low, even with a ton of weight in the trunk.

I've broken adjusters trying to get them to turn, and nothing seems to fit the square pegs that extend off the back of the plastic 90-degree gears. I find that I tend to lose teeth off those 90-degree gear things, which makes it hard to adjust forever. That's something that would be a good target for a 3D printer, actually.

What works well to lube these fiddly things? I was actually thinking silicone might work ok because of how well it creeps along surfaces. Is there a good solution for this?

Happy bricking and I hope we all can spend time with the ones we love this time of year.
Will

The brass alloy thread corrodes and gets stuck in the threaded receiver. The thumb screws can break if you force it.

If the plastic threaded receiver breaks or strips you lose the ability to aim the reflector / lens assembly. I've used motor oil and let it creep in. These may be an embedded in the plastic brass receiver and so corrosion form from exposure and they can seize.

The FAQ:

https://www.brickboard.com/FAQ/700-900/ElectricalLightingHorns.htm#Headlight_Aiming

Some liquid oil where the adjuster threaded shaft meets the threaded receiver.

A little unsure.
--
Give your brickboard.com a big thumbs up! Way up! - Roger Ebert.