I bought some of the Radio Shack gel lube today and applied it to the helical threads on the I-50. It's turning much easier now. Thanks for everyone's suggestions!
As Paul, said, this lube is a little stiff for the job I'm using it for. If I need to do this again on a different lens, I'll probably get a different kind of lubricant. This I-50 is going on the Leica IIf I bought on Ebay yesterday (it's the only collapsible lens I have) and I can live with the focusing ring being a little stiff as it's only going to stay there until I save up some money for an Elmar. Provided, of course, the nasty looking scratch and cleaning marks on it don't result in nasty looking pics. In which case, there's an I-22 in my future.