On one apparently knowledgeable site, I found this recommendation:

I use Permatex Superlube, a PTFE based clear grease sold in a sqeeze tube at, among other places, Radio Shack. A tiny bit, just two or three 1 mm dabs applied with the end of a toothpick, is enough for the focus threads in a front-focus lens like the Agfa uses; it's viscous enough you won't have the focus moving on its own after you set it, but smooth enough that one finger is adequate. And it *stays* slick.