Have your 20mm rail and blocks. Is there a special grease that you recommend for the blocks or is general chassis lube OK?

You can use grease, low-viscosity grease or oil. Your application may need to be considered. Here is a document on the lubricating instructions from HIWIN: https://www.hiwin.com/pdf/lubricating_instructions.pdf Section 4 of this document relates specifically to the lubrication of linear guideways like the ones we carry.