What grease to use for linear rails?
Don't use grease, it is better to use a light oil to lubricate the rods.  A light oil will help flush out any dust and filament debris, grease will trap it.

I've used both light machine oil (like used for sewing machines) and PTFE based spray (Teflon). Grease is thick and will collect and trap dust and particles more easily than light machine oil.



Even high-end consumer printers use light machine oil, e.g. the Ultimaker 3 Extended I got came with a bottle of light machine oil for the linear guide rails. Their advice is to regularly add a drop of oil on each shaft once in a while (how frequent depends on how much your printer prints).