With all-metal configurations of ball joints (e.g. ring & ball, mini ball, Uniball), of course lubricantion is necessary. But, what about ball joints with nylon/plastic sockets? Someone in a hobby shop told me that these don’t need lubricantion—may even lead to slippage and/or premature wear. Any insight would be appreciated.

The plastic cap on a socket is a lubricant of a sort; especially if properly chosen. Make sure you properly tighten the retaining setscrew (not too tight, not too loose).

This is an anecdote, not a recommendation. I don't know the answer to your question.

However. My wife broke the rear view mirror in our older car. It popped the glue joint off the windshield. When I was fixing it, I noticed it was extremely difficult to move either of the two ball joints on the link that supports the mirror. That's why it broke, too much stress on the glue joint because the ball joints didn't move, probably.

They were plastic on plastic joints, and I wondered about the wisdom of lubricating them and maybe degrading them. In the end, I dripped a tiny amount of STP into each joint, and worked them until they were smooth and free. This took a long time and considerable force.

It's been a year or so, and the mirror still adjusts perfectly, very smooth but staying where you put it, no risk of putting too much force on the glue.

I put a spot of heavy oil on all ball joints I service, plastic or metal!!

Eric

I always oil those the same time I oil bearings on my valves

I have played on an Edwards Thayer with the same set of nylon joints for 30 years without changing or oiling them... and I play for a giving so that's 3+hrs a day. High mileage and still quiet and smooth.