For me, the issue is not the lube drying out, but instead the lube swelling the rubber bushings on the lower slider pin. The bushing swells over time, and starts to bind in the bore. I have tried a number of different brake lubes, and still run into the bushings swelling. Currently trying Permatex silicone brake lube, the orange stuff. We will see ...
The bushing is used to dampen caliper rattles. You don't really need it, but on some cars, the calipers really rattle bad. On others, you can't hear anything ...