I suggest you disassemble the lens barrel (the optical unit comes out as a unit) and CLA the focusing helical. There are low volatility lubes availabel at electronic/computer shops. Most people use a white lithium grease, but if you have exceptional looseness problems there is also a clear gel lube at Radio Shack which is much thicker. I have never had to use it on a lens, but it should add quite a bit of stiffness to the movement.
I do not recommend lubing the lens diaphragm unless you want to actually remove the iris leaves and put them back in. In that case, the Radio Shack gel lube placed ONLY ON THE PINS, will both hold them in place during reassembly and make the movement a lot stiffer.
Be careful with the headless set screws. They tend to be brittle and break apart at the slots quite easily.
-Paul