Hey all, I've been lurking on this and the other thread, and I thought I'd chime in. I did this fix a few years ago on a number of old F4's that were heavily used, at least 3-4 bodies, some minor some more serious of a slow down of the lever. All of the bodies were super well used after thousands of rolls doing weddings and while basically in good nick the slow lever action would make inconsistencies and I had really nothing to lose. I did a number of attempts, with CRC electrical cleaner first with operating the camera while spraying thru a long thin straw to try to target the 'joint' where that lever pivots, if you can see the part on an exploded F4 diagram you'll see what I was aiming at. After a good amount of spray/shutter cycle I switched to a more precise method of tracing the lever down with a thin wire and then dripping small drops of lighter fluid (Naphtha) and then cycling the camera plenty of times, doing this a couple of times finishing with lots of spray air targeted to dry the area. When things felt smoother, less sticky, I followed up with the same method of dripping tiny drops along a wire, but with this last one with CRC greaseless lubricant, not much. My theory was from the first cleaning as a general clean for the area, then the Naphtha as a more targeted clean, and then a tiny amount of lubricant. I think, (its been more than a few years, at least 5-6) I stopped doing the first spray after the first one and just did small amounts of Naphtha to clean, cycle until things seem better, then spray air to clean. I'd let the camera sit for a day or more then check the aperture lever operation with known test lens, if there was any slowness on *the first* cycle then I'd repeat the whole process. If and when a camera would pass that test I'd let it set a week and test. I think most of the bodies needed at least 2-3 attempts, and I would test them with film by putting the camera on A and shooting a sequence while moving the aperture ring from wide open to full stopped down. If there was any and I mean any residual lag the negatives would show over exposure, a correctly working lever would give perfectly even negatives which would be obvious when the negs were put on a light table.
As it was/is, I managed to revive those bodies and I passed one or two bodies off to friends/family and kept two or three, I lost those and several newer F4's (and a whole lot of other stuff) in the Lahaina wildfires, but I did save one F4 with MB-20 that I used to photograph the fire and its aftermath. That F4 was one of those that I did the revive on, and its been a regular shooter for me with no exposure errors, but I do try to shoot a roll a month with it.
Of course YMMV, but this worked for me. I now routinely test any F4 I find for sale by using the stop down lever and feeling how smooth it is and listening to the sound, if its too 'creaky' I figure its slow, and offer accordingly. Good luck with the mission, I will be following along.