@spectra: This is a great thread.
I do have a viscous coupled fan in my LX470, and I do keep an eye on it. So far it is working fine. The fan normally starts out a full lock when the engine is cold started and then immediately disengages, so I hear the full fan roar at start-up that then dies down quire rapidly. Then after driving and the engine is fully warmed up, I check the fan speed with the AC on an off. It should be fully locked with AC on, and appropriate to the engine temp with AC off.
I'd like to see pictures of the couplings in your three cars.
Generally, I would recommend NOT fiddling with the set screws and only changing the oil if needed.