After blowing up my second linear rail tonight (I didn't follow @csorrows' advice so far...) I want to share some insights and get a recommendation for the perfect lubricant.
Plus, I would really ask the Folger team to include this knowledge in their owner's manual: Cleaning process during assembly, cleaning agent, lubricant, lubrication intervals. Some lines of text would have saved me a nice amount of time, hassle and money.
So, I now cleaned out my linear rails with professional-grade sewing machine oil. This is how one side of the cleaning tissue looked like:
The other side is just as bad. Everything from thin brownish oil to thick, black, gooey stuff.
For the next print hours, the carriages will run on sewing machine oil to clean out all rust and dirt. Afterwards, I would like to switch to the correct lubricant. In this thread, @csorrows states he uses/used PFTE-based grease.
I did some reseach and the matter seems a little complicated. Lubricant manufacturers provide different recommendations depending on the operating environment and the type and speed of carriage movements. This makes sense to me - for example, during micro steps, the carriage only moves a millimeter or so back and forth. I can imagine that this creates very different lubrication issues than a wheel bearing that always spins in one direction.
In my case, I will also have to take the temperature into account - I plan on converting both my FT-5s for enclosed ABS printing, so the rails will work in 70°C environment temperature.
Thus I would be grateful for suggestions on particular lubrication products - so far I could find that stuff - and also for best practices on how to get the grease into the carriage, lubrication intervals, cleaning etc.