Not sure what the original Hasselfors turnbuckles where made of - I re-rigged in 2015 with Sta-Lok chromed bronze. I used standard 3-in-1 oil for lubricating them during setup and adjustment. Sounds like you might have damaged threads though the way you describe it, possibly from previous galling, in which case you should take the turnbuckle apart (support mast with halyard, obviously), have a look inside and at least replace the turnbuckle body, stud or both depending on what you find.
Your rig is quite old if original and if key parts are starting to act up I'd at the very least have a thorough inspection of all parts and consider replacing it. Insurances tend to get fidgety at about 10 years and really dislike >15 year old rigs.
Selden sells a
rigging screw oil specific for the purpose. If you're going to spend silly money on goop, buy that instead of Tef-Gel, which is ill suited for that purpose due to high stickyness attracting dirt and keeping it in the threads. The oil is applied before making adjustments and washes away in the next rain (bronze fittings do not require anti-seize lubricants as they do not gall).
