I have two and both of them suffer from a sticky diaphragm (iris aperture) syndrome, like most of old lenses.
I read horror stories about this lens, people ruining it completely, declaring it unrepairable or so. I don't think it is any harder than other lenses I tried to repair, provided it is just stuck and not broken. When it gets stuck, the blades gum up very hard so trying to force it may cause the pin and leverages to break.
I find the lens quality quite nice, for being a triplet! Smooth colors, bokeh and only slight vignetting. I disagree with people calling this a bottle bottom glass or "Holga like"! I think people having bad experiences had badly serviced or broken lenses. See the examples below.
Start by removing the back-part, the bayonet. Then you can remove the barrel holding the lens. You do not need to remove the lens itself. Three screws and you get the parts seen in the picture below.
Remove all the blades carefully, put them on a paper towel.
Clean the blades with petrol, light gas, lighter fluid, what you prefer. They are made out of steel, be careful not to knick them and not to damage their drive pins.
If you had a serious oil problem, then the lens surfaces might be hazy and you need to clean those too.
The blades need to be inserted one over the other, like a spiral stair case. The most difficult are the last and the butlast, because the nose needs to go "below" the first, it can be seen well in the picture below.
Try not to touch the blades with greasy fingers. Once all blades are in, you need to slightly close them before putting the cover ring and screwing it down. To do that, gently actuate the leverage, as shown in the following picture. The three screw both fix the upper part to the ddiaphragmring, but also fix the whole assembly to the lens barre.
Once closed and screwed down, the actuator lever should already work smoothly. Don't close it too much or it will stick.
Remount the rest of the lens with care. Take care of the two opposed springs that go into the barrel. Take care that the f-stop-ring is aligned properly.
The Exakta version is easy to remount, the M42 version has a different linkage to the bin, be sure to mount it in the correct open position!
Once everything is reassembled, use the lens! In my experience it is a light, compact lens which works fine with color and b&w films. It works best around 8-11, but 5.6 is already quite fine. Of course at 2.8 it vignettes a bit, but it remains reasonably sharp in the middle,
Here a photo taken with my Exa and the Domiplan:
Here again, wide-open at 2.8: