The preload and the seals and wiper keep the trucks from moving freely. I think what you are seeing is normal - sounds normal from what you have described.
There is always some space between the balls - 3x? I am not sure of that - I don't know what the space is supposed to be.
Grease - use a very light bodied lithium. DO NOT use regular grease - it is too thick and will prohibit the balls from cruising down the re-circulation channels freely.
Scott
Consistency is a good thing....unless you're consistently an idiot.