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Don't worry about the preload it won't be a problem. By hand they feel stiff but to get a better idea of how they will really feel then put a rail on a level floor and put slight pressure on with...

@dezaud,
You might find these threads worth reading.
http://www.mycncuk.com/threads/4862-Profiled-rails-Installation-guidelines-and-manufacturer-comparison-chart?highlight=hiwin+lubrication...