I use a lanolin-based lubricant, Forespar Lanocote, on our
turnbuckles and on the backstay adjuster (which is more-or-less a turnbuckle with a wheel on it). It's pretty stiff. I apply it with a rag--working it into the threads. It's relatively weather-proof. I got it at West Marine. Here's their web page on it:
FORESPAR Lanocote Corrosion Inhibitor
Warning: It's kind of... odoriferous
Jim