I found the best grease to be 'vacuum grease' - made for laboratory glassware fittings. It won't outgas (read fog the inside lens surfaces), migrate (read gooey diaphragm) or separate into oil and sludge (read seized focusing helical and an oily diaphragm). My experience with silicon (Silglide), Lithium (Lithease) and Calcium (Lubriplate) greases is that they do all three given a little bit of time.

You can also use 'optical damping grease' but it seems to outgas a wee bit and fog lenses when the grease is new. It is probably a good idea to bake the grease first to get the last of any solvents out of it.