riverfever said:
Why? Because of the curve?
The windshield is different because it is glued in with urethane, and the donor windshield, if you're fool enough to get a used one, must be sliced out with great skill and effort, or more of the latter to match the deficiency of the former, and the new windshield must be set in with the urethane adhesive.
The old fashioned gasketed windows are much easier to deal with. I haven't tried silicone when doing this, but a little talcum powder on the rope helps (I learned my mad window skillz in the 60's when silicone was not the common stuff it is now). Dish soap also works if you work quickly, but it gets stiff after a few minutes. There's a nice lubricant called "Ru-Glyde" which is used on tire beads, shock bushings, etc., but I don't know whether you can get it in quantities smaller than a gallon. Silicone lube should work pretty well, and silicone grease has a useful tendency to stop small leaks (great for those leaking vent window gaskets).
Whether the donor glass pops out depends a little on the vehicle. A Jeep window that hasn't been tampered with probably will come out easily, gasket and all, but you need to be careful not to push too hard on the glass. Some are tighter, though, and if someone has tried to seal leaks with some kind of goo, then you have a problem. If in doubt, I'd recycle the gasket from the broken vehicle and sacrifice the donor's gasket. You can just slice off the flange of the gasket with a utility knife and remove the glass without any stress.