I repaired two Isolette folders with stuck focusing rings. You really have to remove the front element and carefully clean all the old lubricant out to make it a permanent repair. I used a lubricant from Microtools called "Super Lube Grease" to re-lube. When the front element is removed, it's position for infinity focus will have to be re-adjusted. I use Ed Romney's method (from his camera repair book)to set the infinity focus.
I would be careful about flooding it with solvent. The loose gunk can get into the shutter works and blades.
Good luck, these old folders are fun to use!