There's been many discussion about what type of lube to use. First off DO NOT use WD40, JB80 or similar. These are penetrating solvents. These are not to be used as a lubricant.

The MGN carriage comes with manufacture lube on them. The carriage should slide along the rail with little force required. I don't have the preload information on these but it's not uncommon to have to apply slight downward force on the carriage for it to slide smoothly