I see many, if not most, posts on this topic are in favor of either vaseline or KY, so, keeping with my usual 'gotta-be-different' attitude, I'm jumping right in and saying, "Silicone."
Silcone is safe for use with most materials. The real kicker, though, is it is apparently not safe for use with items made with silicone. I've seen statements cautioning against using silicone lubricants with other silicone products.
It lasts and lasts because it is not water soluble and is very difficult to wash off even with soaps.
It's economical because only a few small drops will coat even the largest nozzle, plug, or other part that needs lubricating.
It must be safe for people, too, or it would not be sold as a lubricant. I'm not a doctor, chemist, or any other type person with intimate knowledge about this, but I've found no articles on the internet discussing silicone as a personal lubricant. Have you seen any?
Things I've used silicone lube with:
* Enema nozzles - machined delrin as well as other plastics
* Butt plugs - steel, jelly, soft plastics
* CB3000 chastity device (polycarbonate)
Now I'm getting horny. Se you later.